Different bits will work on different horses, and it also depends on the rider's hands and experience and reason for choosing to use a particular bit. Okay, that's my warm and fuzzy PC answer.
Personally, I don't really care for the WonderBit, but I have only had a few experiences with them. The one that really comes to mind is also a case of a young TWH gelding that belongs to a man in his sixties. Though the owner has been riding most of his life, this is the first gaited horse he's had - all others have been TBs and stock breeds.
He basically just chose the WonderBit because that's what the previous owner was riding the horse in, and for as long as I have known him he has kept his TWH gelding on a tight rein. As a result, the horse has a very attractive head carriage (if you're into the rollkur look), a nonexistant 4-beat gait, and a temperment that is wound tight at all times when under saddle.
Of course, the bit is only part of the problem. The owner insists on the horse being (his version of) "on the bit," and the WonderBit is good at creating that illusion. I've been on the horse a few times in recent months, and used one of my Robart TWH bits with good results, and also let the owner try it. Of course, he didn't like it. . .didn't give a real reason, just that it didn't make the horse "collected" enough.
There's only so much you can do. He just wants the bit to do all the "work" for him. . .and, IMO, the WonderBit is not the best choice for a rider like that.