It depends how well the blocking features will be advertised. "Lots of extremists have converged at xxx.social. You can block them all in one go." sounds better to me than what's happening in Twitter.
Agreed, federated instance is a way better solution than Twitter blocklists. In fact, I think Mastodon was designed to combat such extremists from day one, so even with the alt-right’s foray into the fediverse things will go at least better than Twitter.