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.
"Allied Force", which perfectly fits its use in Episode 15, chronicling Fellini & Fenice's history together, and their Heroic Second Wind.
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.
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.
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.
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.
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.
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?
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.
Not to mention that Build Strike and Star Build Strike's beam rifles make the classic beam rifle firing sound from the original series.
Narm Charm: Yuuki-as-Meijin has the habit of making very Narmy remarks in all seriousness.
The Scrappy: Nils so far appears to be one of the least popular characters, thanks in no small part to his obvious Marty Stu traits. His Curbstomp Battle against Greco, who's quite likeable despite his small role (and being The Determinator to boot) doesn't help at all. As well, despite being the American representative, Nils is a rather blatant Japanophile, which has bothered more than a few viewers.
Before Nils, there's Kirara. Though Kirara wasn't hated that much as time goes on.
Ditto for Fellini. Some fans didn't take well to his being The Dandy or how his fighting skills were, for a time, totally informed. But then Episode 15 rolled around, and...
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, while his Marty Stu and Up to Eleven Japanophile traits remain, his being subject to the crazy antics of Gundam Build Fighters' setting (namely Caroline's roping him into being her one time henchman and later declaring him her boyfriend) alongside his display of valor in Episode 12 have earned him continuously growing favor with the fanbase. And then there's his performance in Episode 19, which showed that, in spite of his original goals in entering the tournament (gaining access to Plavsky particle technology), he's as much a Gunpla Builder and Fighter as the other characters, such that he gets enraged when his "perfect" Sengoku Astray becomes horribly damaged.
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.)
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.
Tear Jerker: During the fight between Wing Gundam Fenice and Star Build Strike, you see a flashback of Fellini playing and battling with Fenice ever since his childhood, and Fenice's evolution from a standard gunpla into it's current Ace Custom form. It's enough to bring a Manly Tear to your eye.
Mao crying on a riverbank after losing to Sei and Reiji. Made worse by the fact that it's Mao.
Fellini's face when a berserker Aila is destroying his Fenice after Fellini had lost and couldn't do anything to stop her. It is made worse as we saw what they went through from episode 15.
Speaking of which, what happens to Aila in the same episode as above. Already upset that her next opponents would be Reiji and Sei, the aforementioned Mind Rape drives her to a mental meltdown. As if that wasn't bad enough, the look on her face when she sees Reiji with her cracked helmet. After having found new enjoyment and new friends, it's crystal clear that Aila is terrified that she's going lose it all, including her own mind.
Gets even worse in the next episode which opens up with Reiji, not realizing that she wasn't in control of her actions, hatefully yelling and berating her. The reactions she has during this scene are practically soul-crushing.
Aila's life up to Episode 21 is nothing but this. It's revealed that Aila was an impoverished orphan who only garnered attention due to her ability to predict outcomes for Gunpla battles. Any sign of care Nine showed to her during his scouting trip disappears after she is recruited. Being seen as nothing more than a tool for Team Nemesis to win Gunpla battles, she was pressed to being a subject for the Embody System, a dangerous and illegal system that increases reaction time at the cost of consciousness and mental well-being. And it's not like she has much say in the matter, as Nine threatens to toss her out to the streets again if she refuses to follow Nemesis' orders. Thank God Sei and Reiji got her out of this living hell by the end of Episode 21.
Played With in episode 22, where Julian's grandfather, on his deathbed, begs Julian to take his Gunpla and fight one final time with it. Turns out this was a ploy to get Julian to be interested in Gunpla again and reestablish his friendship with Yuuki. Arguably goes full-circle, back into Tear Jerker, at least in-universe, where, after Julian's and Yuuki's friendship reforms, his grandfather is seen crying with happiness.
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.
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.
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.