the immediate one will be that narrowing down relay selection (i.e. relay set) is hard/annoying, so people hardly ever do it and just add "yet another relay", which breaks the balkanization/specialization of relays.

this one way I've approached solving this, which allows for a complete context-switch in a single click:

e.g. you could be using "general" nostr and one-click into bitcoin-park nostr, and then one-click into you-and-your-families nostr; with different profile info, different client settings, etc

nostr:note1e95enlnpu5prnt4j5gjucm4m8rrqzqz6eh22t3rh896v9rdwen5qq8nysz