So after being disappointed twice, that is, I got to play two games I was really hyped for (Battalion 1944 and Insurgency: Sandstorm), only to find out Mouse Accel is disabled by default. I noticed both those games were under Unreal Engine and that seems to be the reason.

So, I've read about tons of posts on how to disable it in other, older UE games (KillingFloor2, PUBG), but using their solution in reverse doesn't seem to have effect (bViewAccelerationEnabled=true). So either I'm using this wrong, or it's just not editable atm (or it's now how to do it at all).

What I want is very simple: To have the windows desktop Mouse Accel keep happening in-game. I have it in the menus, I can see it on my cursor, but as soon as I enter first-person view, it's gone. I think it has to be because UE uses a different method to move camera than most games? Every other FPS I play (original Insurgency included!) has Mouse Accel in-game. I wouldn't mind having it off by default since I seem to be in the very minority here, but y'know, just allow it to be turned on, pretty please.

So to be clear, I'm not using any special mouse acceleration software or special mouse, I'm simply using Windows' "Enhance pointer precision" setting. Everything else is bare default. It works great in any game except those UE ones.

My specs:
Ryzen 7 1700 @3.0Ghz
16gb RAM
2x GTX 980 SLI
Windows 10 Pro 64-bit

My mouse:
(I'm as surprised as you that this mouse plays so well)

~Ending note~
Before anyone suggests it, no, I won't stop using mouse acceleration because my aim is already excellent and consistent with it, and I won't completely bust my aim in every other game, just for one game (now two). It's really a matter of habit and preference, and I'd even argue that using it is a tiny bit superior, because it lets me do 180s very easily and still retain deadly precision. But then again some pro Counter-Strike players have shown it's also "easy" without it. It really is just a matter of feeling. To put it quite simply: to each their own.

Thanks for reading and considering this.

(By the way I've played a bit and, apart from the optimisation issues everyone else has, this is already very great. A clear improvement on the already great original! Now I just with I had proper aim to fully enjoy it!)