I have a registered 11.5" gun. A Bushmaster barrel and upper. It runs super-reliably. There's nothing special about the gas tube, gas port size or anything else.
Most 11.5" semi-auto uppers will run reliably with no work at all. There are certainly exceptions to this, of course. Full-auto reliability is more of a challenge, but usually a "pig-tail" gas tube will fix that. No downside here except in the odd case.
Sure, ballistics fall off after 100M, but in my neighborhood there are few places that you can actually see that far unobstructed. For anything less than SHTF work it would be extremely unlikely that you would need to shoot further than that given the requirement for target ID. My weapon shoots accurately out to 200M, although ballistics out there are indeed questionable, not that I would want to stand in front of those bullets. For most purposes, again no downside. If you need to shoot past 100M, well then you've got an issue.
Yes, it's a bit louder, but only for the bystanders, not for the shooter! With a Vortex or Phantom flash suppressor and good ammo there is no flash. No downside here.
I wouldn't trade mine for the world. It is incredibly handy and light. It's easy to store. It's much easier to use indoors than a 16" AR or an 18" shotgun. It's going to be more effective than a pistol. With a sound suppressor it's no longer than a 16" weapon. If you're not shooting past 100M what's not to like?
Lately I've been thinking about putting together a 10.5" FAL. Now that would by handy...