Hypnotism is real and it works, but it isn't anything super powerful. It is a suggestion and the hynotee will either choose to follow or not depending on numerous factors. It's power lies in the fact that it seems to go straight to the subconscious, a region which we typically can't access very well...whatever that really means.
the point is, it is more powerful than a normal suggestion:
EG: Me walking up to you and saying "Hey, why don't you stop biting your nails"
but is essentially the same sort of thing.
Perhaps it can work to counter nail-biting on acount of the fact that Hypnosis is supposed to be subconsciousstuff, and nail biting is subconscioussort of stuff too. So me telling you to stop biting your nails is one thing (You may agree with me and tell me that you are trying to stop), but then you subconsciously continue to do it when you aren't aven aware of it. While Hypnosis tells the part of the brain which actually does it to stop...?