As to flyers, see my last comment.
Stubborn doesn't help for pinning. Yes, truly fearless armies, this will be a problem, but, as discussed elsewhere, the big issues are going to be found with Eldar and Tau. Tau are pretty non fearless, and actually have pretty low leadership, forcing riptides to take a pinning check on a 5 (-1 from near a lictor and then -2 for The Horror) is pretty hilarious. Obviously fearless vehicles are a problem.