No downers on the thumbhole stock whatever.
I've been using one on a walk & stalk rifle for big game, varmints and predators for over 30 years.
My LRH rifle has a thumb hole and my 338 RUM is about to.
The biggest plus is that if the thumb hole/pistol grip is properly fitted the trigger finger comes straight back under almost all conditions.
Also, I shoot everything single shot and notice no handicaps with the thumb hole.
I feel that the key to things is the 'fitting' of the thumb hole and pistol grip. Be careful material is much more difficult to put back than remove.
As was said above, personal preference plays a big part in the this.