Being old school myself I preferred traditional over thumbhole look and aesthetic -
However the ravages and challenges of time
(both injury and arthritis)
has drawn me to what works.
Even with very large hands I want to enjoy and accuracy with magnum pistol loads - especially for hunting
The ergonomics of thumbhole with the added loop grip affords me advantages to do so
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.
I use a thumb hole stock on my Contender frame as it gives me a better cheek weld for a scope and I like thumb hole stocks on rifles anyway. I dunno about the one for a pistol. It looks cumbersome. If I were able to try one out I might change my mind.
If you get one and don't like it I bet Marty would make it right. Having dealt with him in the past I found him to very nice and helpful.
However, as a single shot, if a quick follow up shot will be needed would perhaps be the wrong choice
I have used my encore muzzeloader sucessfully on whitetail for years without wishing for a second follow up shot
Due to arthritis and injury I favor cocking a hammer, whether Encore or Single Action Revolver, with my offhand anyway and would not be additionally restricted by a thumbhole in my experience