Well the other thing here is that people still expect the DMR to behave like the BR. The BR was like a pistol, a rifle, and a sniper rifle in one gun. You could fire it at almost any range, in any situation, and get away with it. If someone rushed you in a confined space, you could reasonably expect to win if you shot better. In Reach you'll probably lose, and that's because you shouldn't expect to consistently win with a rifle in close quarters against a guy who's straight-up pounding you in the face. But you manage the situations you get into: you decide when to be in a 'close quarters' space, you choose whether or not to switch to pistol, etc.
I mean, you're a better player than I am and play both games to a higher standard. And in H3, if you could think quicker and aim better with the BR you were going to win in 90% of situations. In Reach, a lot of the time you just won't have the tools for the job, or they'll have something that trumps you (Armor Lock, say) and there's nothing you can do. I can see why that pisses people off. Especially people who were very good at H3 and used to being able to shoot their way out of most situations they got into, even when they made a bad decision about where to go and when.
So many things in Reach, right down to the slow shield recharge, demand you be a lot more careful and almost cowardly at times, and metamanage the game so you're in good spots more often than not. When you mess that up and end up hammering on your DMR point-blank against a guy who just came around the corner and punched you... well, the problem isn't that you can't just outshoot the guy anymore, it's that you walked into that situation in the first place. And no one can help walking into shitty situations like that several times a game. And it can feel arbitrary and random and punishing.
I find I spend a lot more time thinking about map movement and trying to guess what people will have and where in Reach than I did in H3. I just don't have the dexterity and skill to always do what I want to do in response, and things like bloom and decreased autoaim can make that very frustrating.
But I mostly blame myself for making bad decisions and having worse execution. (Most of) the systems are fine to me.