I tried a brace on an AR-15 pistol and found it did not suit my needs.
I have a couple T/C pistols, one with a 10" barrel and the other with a Super 14" barrel. I used Pachmayr stocks on the guns and they worked well. I'd rest the fore end on something to steady the gun. The 10" can be shot off hand fairly well but the Super 14 is a but unwieldy shooting off hand.
I have two Contender carbines, one 221 Rem Fireball, the other 22 Hornet. Both have have standard style stocks on them but a thumbhole stock would probably be ok. The 221 Rem FB is one of the most accurate rifles I have. Bullberry barrel and Bullberry furniture.
But that is my opinion. Others may have different experiences.