the thompson, give me two more plz
but yeah, there is literally 0 drawback to 30's, whereas as mentioned, the thompson can be situational, i only prefer them 'cos they make a cool sound and can wipe squads if you get a really good flank, which is fun. tactical assault is cruel if you can managed to jump some low health retreating squads. also, 3 survival instinct bulletins can be a lot of fun.
There is a glaring drawback to .30s compared to Thompsons: dropping them for the enemy.
The Thompson "focus fire" or whatever it's called ability will melt just about anything on two feet if you activate it in close range.