Full-auto is no-brainer for assault rifles. There aren't many reasons to use SMGs if you have the str for rifles. And if you have a max dex build, you probably can't afford the str.
Commando is also much better for assault rifles since their bursts are much stronger (higher base damage, muzzle brake, full-auto, concentrated fire) but they get less of them. SMGs really don't need that one extra burst, they already have too much.
Muzzle Brake on weapon is a no-brainer for both.
Rapid Reloader is also excellent, but I figured it would be too much for my particular build. (Read: I couldn't find any and didn't want to start savescumming.) If you have max dex and blitz, you could get too many
bursts, up to 11 per turn. Normal 5mm SMG magazines can only handle 8 full muzzled bursts without reloading or 10 with XM.
I honestly thought max dex SMG character would've been totally out of line with expertise and muzzle brakes, but I guess not. Barely doing a third of the damage/AP certain other builds can do, with easy-to-resist small numbers of mechanical damage, and at unsustainable ammo cost. It's still stupidly powerful compared to what the same character could do in previous versions though. Expertise about doubles your damage, muzzle brakes are a 40% increase on top of that and Blitz got me 2 extra bursts.
Ammo availability (and magazine capacity to a lesser extent) keep high dex SMG builds in line very nicely, no matter how many bursts you can get per turn. Vendors couldn't keep up with my ammo demands before, now they haven't got the slightest chance.
In conclusion, SMG Expertise builds are surprisingly balanced even when taken to their extremes.
PS. Blitz sucks with the limited conversion. Any character meeting its 10 agi requirement will have more MP than it can use.