Author's Saving Throw - Fans began discontented grumbling soon after notorious director Takanobu Terada announced R would be limited to approximately three mecha per series. In an impressive turnaround time, Terada made a second post the same day, explaining any mecha with upgrades (such as the Lancelot Conquista and Albion) or alternate equipment setups (such as the VF-25F Messiah) would be considered a single unit for the purposes of headcount, meaning that "three per series" was true From a Certain Point of View. Of course, fans aren't too upset, seeing as this means there will be more playables than previously anticipated; for example, Code Geass has six machines (the Lancelots, Guren Flight and SEITEN, Shinkirou, and C.C.'s Akatsuki), and Macross Frontier has all five primary pilots (Alto Saotome, Mikhael Blanc, Ozma Lee, Luca and Klan)
They probably went a little two far, though: Lancelot Conquista and Albion? Great; Guren Flight and S.E.I.T.E.N. Eight Elements? Also good; Crossbone X-1 Kai and X-1 Full Cloth? Very good. Then it gets worse: the original Guren MK II is passable because you play it for a minute so that they have an excuse to use the missile upgrade sequence from the anime, but the original Lancelot and Lancelot Frontier? Conquista's far superior! And the Crossbone X-1? That's just X-1 Kai, but without the Screw Whips. Sad...
Don't forget about Souske's M9 Gernsback, it has identical equipment to the Arbalest minus the Lambda Driver and its Specials, so its only really good if you want to play without spamming Powerful shots.
At least many of the unlockables are really good: Klan in a skin tight suit with a VF-25's FAST pack strapped to her? Both cool and canon; a playable Nineball Seraph? Just AWESOME.
Terada still did lie about one thing: he said R would use instrumental SRW-style themes...which is true, but the game features a background music editor, which means you can bring in the orginal openings and soundtracks to the series. Fortunately, this little addition is almost universally praised.
A.C.E. Portable is seen as this, since it comes right on the heels of the critically panned R, was at 90% when it was announced (presumably owing to the fact that almost 90% of it is recycled from past games) and it does away with elements of R that were unpopular, like the Tension-based weapons set-up and the Rail Shooter segments.
Breather Level - Subverted in 2, where a back-to-back set make up the game's final two levels
Also subverted earlier in the same game, where the player must play through two back-to-back levels: one involving an Escort Mission, the other involving a boss battle, who you must face with damage carried over from the last mission
Game Breaker - The "Limiter Removal" option allowing infinite ammunition once a mech's stats are upgraded to max. It's especially bad for the likes of the Wing Zero Custom and the two METEOR units, who can clear entire maps with ease when they don't need to reload
The Buster Ark's Riot Shot attack in 2 which, due to a Good Bad Bug, can literally One-Hit Kill any enemy if used at point-blank range
And of course, the Nirvash typeZERO Spec 3 in 3, easily the most powerful unit in the game (and indeed, is a Bragging Rights Reward for clearing each and every hidden mission). It only has three attacks/weapons, two of which are on par with the RX-78-2 Gundam's Beam Rifle and Wing Zero Custom's Rolling Buster Rifle, and a third which prevents nearby (non-boss) enemies from attacking for a short while. Couple all that with reduced damage compared to other mechs, and moderately swift reload/recharge times, and you have a playable unit that can make even the Final Boss run home crying for mommy
That line about Spec 3 being the strongest? That is true... to a point. Get Turn A Gundam to a Limiter Removal, and it gets an infinite supply of hand grenades. Doesn't sound like much? Well these hand grenades happen to actually be Nuclear Warheads! Combine that with the Moonlight Butterfly, which does insane damage to all units anywhere close to the Turn-A, and its extremely potent Healing Factor, made all the nastier by the really high armor rating it'd have by then, and you have a unit that can beat even the Final Boss in a matter of seconds.
One of the ART-1's Tension attacks can one shot almost all large Mooks if you hit the right place. That, and it's a Lightning Bruiser. In fact, were you to max Ryusei's stats early in the game, the rest of the missions makes R a joke.
In 2, Altron Gundam. Its Dragon Fang attack does damage on par with Wing Zero's Twin Buster Rifle, but it reloads much faster (as in, as soon as the fang returns to Altron's arm, it's ready to go again). The TBR has better range, but Altron can single-handedly tear through just about any enemy in the game; the main exception is Large-sized enemies like mobile armors and battleships, since the Fang doesn't work on them. Of course, it's still a pretty good machine even without that move.
Crowning Music of Awesome: To be expected, since many of the units use memorable themes from their respective series. Shinku, the main theme of ACE 3, also counts.
Good Bad Bug - R features a handy little trick that'll let you max all mecha in the game as soon as you can upgrade a single unit a single level, legitimately. Characters require at least 1000 experience points each to do the same trick, but it's possible to maximize just about every single character in one playthrough.
It's Easy, so It Sucks - A common complaint about The Final is it's significantly easier in difficulty than its predecessors.
Scrappy Mechanic - R's Tension gauge system; replacing the ammunition-based combat system of the previous three games, R's system gives your standard weapons infinite ammo...but anything other than your basic gun, you have to build up Tension in Limit Break-fashion, then expend it to perform your bigger moves. Also applicable to R's boost system, which reduced the speed of the game (except when you need to get somewhere fast, in which case you're a runaway freight train).
2 has two (no pun intended): the Neue Ziel, which only takes 25% damage from any and all Energy Weapons (it's the only boss battle that separates into two missions and you only need to damage 50% of its full HP in each), and the Buster Ark due to general hax-ness.
3 has the Nirvash Type-theEND, which is insanely fast, will deflect any and all missle attacks thanks to its chaff, and can Beam Spam you to death fairly easily
R has the Bird Human. Who the hell sics a 10-kilometer wide boss with a 15-kilometer wide attack at you!?
R brings us "The Two Divas", a level that has been tossed onto the scrappy heap by fans not for being difficult, but for consisting of nothing but Swift Chase segments interspersed with lengthy, unskippable cutscenes and ending with a Cutscene Boss.
They Changed It, Now It Sucks - R generated harsh criticism in Japan for removing nearly everything about the older games but the crossover nature and action-oriented style, fixing what wasn't broken
The staff had stated outright that R is an Obvious Beta. At this rate, there won't be a release version.
This has become a bit of a complaint concerning R, as well: of all the included series, only Macross Frontier has its plotline resolved within the context of the game. The other series' denizens are simply sent back to their original worlds in order to work out their differences amongst themselves.
Wangst - Barrel goes through several bouts of this alongside Renton in ACE 3.