The rest of the firearm configuration determines what the barrel device can or cannot be.

For rifles:

Featureless cannot have flash hiders, as that's one of the features. Muzzle brake, threaded with a protector, non threaded would all be okay.

If it has a fixed mag, it can use any of the above, or a standard flash hider.

Doesn't need to be pinned unless the barrel is shorter than 16" and you're trying to make 16" with barrel and muzzle device combined.

It's up to the owner to configure their weapon to comply with the law. I can buy any standard threaded barrel and I can put any muzzle device on it, the legality of it is only determined when I attach it to a lower receiver based upon the configuration of the lower receiver. When it's not attached, it's just parts.  

Pistols:

Threaded barrels are not allowed for pistols. I believe a permanently attached brake or permanently attached thread protector would be legal on a threaded pistol barrel, but there's a lot of other complex issues when it comes to AR type pistols. Again, it's on the owner to configure their firearm in a legal manner.

I am NOT a Lawyer and none of this is legal advice.

And someone will probably come along and explain it better than I did.