Two concerns come to mind:
- Collateral damage, or the threat of hitting innocent bystanders either with ricochets or stray rounds.
- Loss of the weapon as a direct consequence of a shooting, while being held as evidence. If this happened, you'd lose a significant investment.
An SMG compact enough for concealed carry is very short, and a challenge to accurately shoot. I have an old SWD M11/9, and I need a *lot* of practice to stay proficient with it. I would never carry it outside of a situation similar to the 1992 LA Riots.