The critical detail is that Nostr clients are supposed to be thick and relays thin. That means that problems should be solved client-side as much as possible, including dealing with relay downtime. So, if you want maximum reliability of communications with your friends, you and your friends would use clients that talk via multiple relays (not necessarily all relays at once, perhaps trying one relay at a time until the message gets through). And each relay would be none-the-wiser.