i think their main point lately is that one shouldn't specify too many relays in the NIP-65 metadata (inboxes and outboxes), because that's information for other people and makes it harder to find your posts or send a post to you
apart from that there's little reason to not use as many relays as you want, though things might become slower and/or more battery and bandwidth usage, but doesn't affect anyone else
Battery life definitely gets worse with more relays but things slow way down with less relays…