These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Gundam Build Fighters
Anticlimax Boss: A bit. Even though the battle between Reiji and Sei, and Mao was hyped up for a good chunk of the show. Their final battle was a bit of a Curbstomp Battle. Justified a little since Sei had made numerous additions to the Star Build Strike since their imagination battle, when it still was only the Build Strike Full Package, while Mao didn't really have any additional improvements to his Gunpla aside from the Maoh Sword.
The 2nd opening, "wimp" by BACK-ON and featuring Lil' Fang from FAKY is filled with meaningful lyrics, and the opening animation matches the song well.
Both of Tatsuya Yuuki's themes, The Crimson Comet ~ The Usual Flamenco at Three Times the Passion, and its remix Meijin ~ Six Times The Passion Of the Ordinary Flamenco, both because of the rapid tempo, and for using Gratuitous Spanish:
"Allied Force", which perfectly fits its use in Episode 15, chronicling Fellini & Fenice's history together, and their Heroic Second Wind. Also used during the climax of Reiji vs Nils battle.
When "1/48" plays, you know shit is about to get real. This perfectly fits the intimidating nature of the 1/48 Mega Size Zaku II. It's also a striking track during the first half of Reiji vs. Aila in Episode 21.
"Aila" is nothing short of a beautiful, emotional theme for said character. The heart-wrenching tone of the song works incredibly well when Reiji learns the truth about Aila.
"Power Resonance" - The theme played during the Reiji vs Mao fight.
Chairman Mashita is pretty divisive as well. Some really don't like his Manchild tendencies and his increasingly petty actions against what are basically a couple kids. While some love his exaggerated expressions, especially when they involve encountering Reiji. May be an example of Love to Hate.
Boring Invincible Hero: Star Build Strike becomes this after episode 15. While the Sei and Reiji team making it to the final fight is to be expected, the way each of their battles go after the duel with Fellini is very underwhelming. Once Star Build Strike activates its RG System and launches the Build Knuckle, nothing can stop it. Nothing. It doesn't help that Build Knuckle is relatively boring for a Finishing Move. It's somewhat justified since the Absorb/Discharge System is no longer useable (as everyone and their grandmother knows its weaknesses), which means the RG System is the only super attack that the Star Build Strike has left. Still rather jarring however, especially when the Star Build Strike's main selling point (such that it's a standard feature with its model kit) is its horribly underused Plavsky Wings.
Broken Aesop: The show goes out of its way to point out how wrong the Renato brothers' use of tactics and hidden tricks is, when other competitors fight in similar ways (Aila has the same habit of blowing her opponents up, for example) or even manipulate the Plavsky particles and get a free pass. Hell, when Sei uses the Amuro trap, he gets praised for it!
They are actually criticized because they see Gunpla battles as war instead of having fun at it; it isn't their strategy that is criticized, it's their way of thinking. Nobody has any problem with their tactics, but with their intent.
Doing It for the Art: Mr. Ral explans to Reiji that not everyone builds Gunpla to battle, but to represent their love for Gundam and anime like his HG Gouf that he doesn't use in battle, and to reenact scenes from the franchise.
Aila's popularity exploded after Episode 10 showed that she was Not So Stoic and had Hidden Depths. It also helps that she's the equivalent of the series' female Newtype lead, who have traditionally been very popular among fans.
And now Chairman Mashita's secretary joins the club due to her attractiveness and ability to deal with Mashita's childish behavior.
The little girl shown with Sazaki in the last episode (presumably his sister) became disproportionately popular on /a/, with many posters hoping she gets more screen time if a second season ever airs. Note that this character appeared for only several seconds and received no name or characterization.
Also, she had a tiny Gyan hat. How is that not utterly adorable?
For the inevitable Gundam Yaoi Fangirl demographic, there's Sei/Reiji, helped along by their Ho Yay in-series.
Rinko Iori/Mr. Ral emerged from their status as the only two adults with a significant role in the early parts of the show, the latter's (unrequited) canon crush on the former, and the fact that they spend more screentime together than Rinko does with her actual husband.
Nils and Caroline also had a bit of a following even before the latter declared herself the former's boyfriend. Naturally, this is due to Caroline being the one person who can reign Nils in (and how) and force him (the resident Only Sane Man) to take part in the series' crazy antics.
Allan/Meijin seems to be the most popular ship involving Meijin Kawaguchi aka Tatsuya. It helps that unlike the other main characters, Meijin doesn't get a female love interest and Allan in some ways serves as The Not-Love Interest for him.
Fridge Brilliance: According to the in-show profile, Greco Logan is twenty-seven years old and has been a Gunpla modeler for twenty years. If you take into account this series' probable setting (somewhere in the late 2010s or early 2020s), Greco could possibly be a Winger, an American Gundam fan who literally started out with Toonami's airing of Gundam Wing back in 2000. And considering his build, skills as both a modeler and a fighter and overall Badassery, one could almost say Greco is vindication for such fans, who were heavily discriminated against during the period.
Bonus points go to him using a customized Tallgeese while his Friendly Rival uses a Wing Gundam.
Nils Nielsen's being able to control Plavisky Particles so well makes sense due to his PhD in Particle and Energy Physics and that he has been studying Plavisky Particles for some time, which is his motivation for taking part in the Gunpla Tournaments in the first place.
Harsher in Hindsight: Aila's love for food is revealed to be at least partially based on the fact that she was impoverished as a child.
Ho Yay: Between Sei and Reiji, "partners" in Gunpla battling.
The first opening theme doesn't help much.
lyrics: From the minute I felt you in my heart, I was sure my dreams would come true.
Sei exhibits Tsundere tendencies when dealing with or talking about Reiji.
Honestly, does Allan really have to follow Meijin everywhere he goes?
Interestingly enough, in the last episode there's a montage of all couples (Nils/Caroline, Fellini/Kirara, Takeshi/Rinko, etc), living happily one year later. In-between all those, there's a shot of Yuuki and Allan working with kids.
Marty Stu: Nils Nielsen, in the same vein as a newbie's first RPG character. Holds several PhDs from "America's top university", black belts in karate and judo (courtesy of early training from his martial arts master mom), has only three months of Gunpla building experience yet builds a top of the line model that completely decimates a Gunpla built by a 20 year old vet, and he performed the latter boasting of his superiority and making light of his opponent's attempts at fighting (even though it was clear said opponent, Greco Logan, was the more skilled fighter). Oh, and did we mention he's only thirteen years old, such that his nickname is the "Early Genius"? Combine that with the Anvilicious samurai theme for both his Sengoku Astray (one imagines even Lowe Guele would have found the design over the top) and himself (he even fights in a gi and hakama), and one has a disturbing feeling that Nils is a huge parody of or a Take That to the Western fandom at large.
Moral Event Horizon: Josef Kankaansyrjä wastes little time crossing this in Episode 20. While it's one thing to take your grandchild out to see Gunpla fights, it's another to have Aila's handler raise the power of the Embody System to max output. For the sake of entertaining his spoiled grandson, Josef mind rapes Aila to the point of hitting a mental meltdown and utterly destroying the Gundam Fenice after Ricardo concedes the fight. Worse still, he nonchalantly states that he has replacements should Aila fail from the device...
Nine Barthes crossed this before the tournament began by designing and producing the above-mentioned Embody System. This moment proves beyond all doubt that Nine views Aila not as a person so much as a tool for his company to win tournaments.
And then he's hired by PPSE to develop a version of the Embody System for the Meijin.
Not to mention that Build Strike and Star Build Strike's beam rifles make the classic beam rifle firing sound from the original series.
"Beginning Plavsky Particle dispersal."
Narm: The Meijin's over the top wardrobe change at the end of Episode 23 had many fans finding it difficult to take him seriously.
Narm Charm: Yuuki-as-Meijin has the habit of making very Narmy remarks in all seriousness.
Rescued from the Scrappy Heap: This series seems to have a good track record of making every recurring character that was originally detestable likable. Examples include Kirara and Nils.
In Nils' case, he has steadily come out of his shell since his debut. Getting involved with some great gags and antics with the colorful cast, as well as developing to show his valor and fandom of gundam and gunpla battling, even so much as losing his composure after his beloved Astray took heavy damage. Not to mention his consistently awesome gunpla kills.
For all his character flaws, Fellini's fight with Reiji and Sei in Episode 15 showed that he is one helluva Gundam pilot and Determinator (even more so than his other rival Greco Logan). Especially given this sequence◊. Plus, it also helps that Fellini has transitioned more into The Mentor as the series progressed, such that he's one of the few people who has a degree of influence on Reiji. This came to a head in episode 21 when Fellini baited Reiji (who was still anguished over Aila's "betrayal") into a fist fight so that Reiji could vent his rage.
Then, after calling Fellini out of his self-detonation attempt in episode 20, Kirara is again getting flak from some fans (thus subverting this trope), mostly for having denied a supposed send-off for Fenice (and probably a homage to Wing, too.)
But thanks to her, Fenice comes back a few episodes later with a cool redesign, just in time to take part in the final battle!
"Stop Having Fun" Guys: Yeah this show does get a bit serious with it's toys but most of the characters play it in good fun. A couple do have this mindset however such as the Renato brothers and Meijin and only seem to care about winning. However, in the later's case, it's a ruse to subvert the image set upon by the second Meijin, who believed in this, and show that Gunpla is supposed to be used for fun.
Strangled by the Red String: Averted with Fellini/Kirara and Aila/Reiji. Unlike many installments in the Gundam franchise, Build Fighters takes its time to develop the relationships and has both couples going through highs and lows. For many fans, this makes their respective pay-offs (Episode 20 and 21 respectively) all the more heartwarming.
Tainted by the Preview: Exia Dark Matter and Miss Sazabi were not well received when previews of the high grade model kits were shown. Subverted later on when the fandom warmed up to both mobile suits when the animation version of both suits were shown to have much more detail than their model kits.
Truth in Television: In many Gundam Vs series, the quintessential Gundam arcade game, skills are the most important, and the most powerful suit have such a high handicap that it's better to use mid-tier suits.
On the specific character: Nils Nielsen, after his overtly embellished debut in episode 8 causing controversy, episode 9 prove that even the resident Marty Stu is not immune to zany situation they're not accustomed for, made him fairly more likable. And later into the series his weaboo tendencies are toned down in favor of him being the eternally-exasperated straight man.
Wing Gundam and its model, specifically the original 1/144 version. Seeing how Fellini basically got a base 1/144 Wing Gundam which is not even completely HG, and keep using Fenice and repairing it through out at least several decades even though, better models, have came up brought many Manly Tears, and also caused many fans to dig out their old/forgotten first Gunpla.
The series also did a good job in restoring the franchise's reputation following what happened with Gundam AGE.
Vindicated by History: Look at the comments on the episodes on the Gundam Info channel, particularly episodes heavily featuring suits from Wing. Wingers (who are typically smacked around by UC fanboys) are coming out of the woodwork, thrilled that Sunrise/Bandai are featuring suits from the series. And these aren't people who were fans of Wing for the bishies, but rather ones who liked the action and the mobile suits. While UC STRONK does happen in the random battles, if a named character is piloting an AU suit, it'll usually do good. Build Fighters is something of a vindication for AU fans in general, as seemingly forgotten/abandoned series are getting attention again (who knew a battle between a custom Wing Gundam and an upgraded Strike would be so awesome?).
In a strange way, SEED fans, who have also been smacked around since their series' time in the sun, are also getting some vindication. Yes SEED suits are getting blown away left and right by UC STRONK, but this is evened out by the main Gundam being a Strike and one of the rivals piloting an Astray. Squee.
For funsies, let's see if we can name all the AUs and non-animated UC side stories that have shown up: SEED, 00, G , Wing, X, Sentinel, Blue Destiny, Turn A, G Saviour, Crossbone... with special guest appearances from Detonator Orgun and Aura Battler Dunbine.