Shotgun-wielding Mediums and Minigun-wielding Heavies can fill an assault role easily enough.
But only until they're out of ammo.
My point is that when you're in a fight or interacting with an objective, you'll lose your Disguise - if you're operating near enemies, then you'll probably have to Disguise to perform your task, but that's part of the mission, not a separate step.
Operatives have an ability to hack from a distance, though. This will help them in a lot of circumstances.
AFTER you're done, you'll then need ANOTHER Disguise to get back home safely.
When you're done, you were already successful. Can just as well die like a hero.