"ACAB applies to Garrus"
I've heard this independently 3-4 times over the past week, and it strikes me as such an odd thing to say. Not because it isn't true--we know Garrus wasn't above working a suspect over if he thought it would make them talk. Like, he wasn't just a cop, he was kind of a dirty one. Not in the same way as someone like Harkin, but definitely in the "you better hope his hunch doesn't lead him to you or you're getting beaten with a rubber hose until you tell him what he wants to hear" kind of way. Which is arguably just as bad, if not worse.
No, the reason it's such an odd thing to say is that ACAB honestly applies to about half of the characters in the visible universe of Mass Effect. It's a very "save us, military industrial complex," sort of narrative in many respects--up to and including the part where all the politicians and diplomats basically have "beta cuck," or "dick dastardly's understudy," tattooed on their forehead with very few exceptions.
That's just something you have to accept if you want to enjoy the series. It's a star war, not an insightful commentary on power structures and the abuse of the people therein.
If you want to evaluate it as one, then there are quite a few bigger fish in this particular pond. The Citadel Council alone is one of the most abusable legislative mechanisms conceivable, and admission to their ranks is predicated solely on approval by the current Council. The council whose individual votes would be weakened by adding another member. Not to mention that the idea of an individual speaking for their entire species is bananas on its face.
And not to put too fine a point on it, but Shepard is a fed. Like, a "clandestine intervention and special operations" kind of fed. ACAB absolutely applies to them too.
The Point™: The Mass Effect universe was created solely to facilitate a role playing game in which the player had more narrative freedom than was typical of AAA titles at the time. If you apply any degree of knowledge regarding sociology or political science, the thing falls apart faster than the M-44 Hammerhead. Basically anybody who has spent more than five minutes thinking about it could tell you that. Anybody can also tell you that if the game mirrored an effective and equitable political process, there probably wouldn't be much call to splatter some faceless space pirate against a wall with your dark energy mind powers. If you want to be all cinemasins about it, that's your call, but I don't think you would make a very good action game going about it that way.
I'm not trying to say that you're wrong if you don't like Garrus. It's a matter of opinion, first and foremost. There are valid reasons to dislike him. Like his elevator conversations, for example. But it's more than a little disingenuous to pretend he is uniquely or egregiously problematic in his abuse of power while we control Commander Shepard--the literal avatar of abusing their power with little to no consequences.