I've used both can and airbrush varnish for airbrushed models and they both work great. I do prefer doing it through an airbrush simply because I don't have to go outside to do it, don't have to worry about the outside temps, and it may just be in my head but I feel like out of the bottle of varnish I get for the airbrush that I'm getting more coats for my money than I do from a can. Again, that just may be in my head.
I've never had a problem of the paint peeling off after being varnished. (I've chipped it from rough play, but that's different). The primer (IMO) is the key. If you don't get a good coat down, then the thin AB layers are more likely to peel.
Best advice when it comes to airbrushes (or anything) is trial and error. You can read and watch videos all day and it'll give you a great head start. But actually doing it yourself is key.