they're only viable until you're facing off against someone with body armor and a larger caliber gun - which is why carbines exist to take the place of the SMG as a more portable firearm while having a higher caliber to neutralize hostile forces also in body armor and to ensure greater possible range of engagement even with a cut down rifle unlike the smaller pistol rounds seen in SMGs - then you're boned.

SMGs have their specific and specialized roles in engaging soft targets with less penetration risk than a rifle round, limiting them in their usability.