I don't know about the balance of them. All I do know is that whenever I fought a human in the past they used gryphs. Not as much anymore in TFT though, probably because of the magic attack.
I think the reason they might have made the change from piercing to magic is because they wanted to play down the "airships". I believe that all the tier 3 flying monsters were given magic attack now (frosties, gryphs and chims).