In mastodon, your instance can stop you from seeing content from other instances and ban users from other instances. While this moderation might be nice sometimes, I’d rather it be opt-in than mandatory. Nostr relays don’t have this power. Likewise, Mastodon instances can stop their followers from following you. Nostr doesn’t allow this.
This is exactly why I won't use Nostr. What you're describing here isn't ideal for many folk that are part of marginalised groups. When each individual has to individually block every bigot only after being exposed to their bigotry, then the vulnerable folk don't hang around. This is doubly the case when there is nothing stopping the bigots from just creating another account after burning their first one.
One question that fediverse needs to solve is: how are we going to fund hosting costs for instances and more broadly, development?
This is also something that activitypub communities do better, because they are communities not relays.