This is not a good thing. It also means there's no consistent profiles across apps. Which means no workable UX for micro apps. Lightning payments will fail and be inconsistent. One relay going down will disrupt the user experience and many leave. Followers and profiles get lost all the time. New users cant get started with a single follow. The idea of nostr as a multi app decentralized eco system is currently completley broken. We need opensats to realize this and help fix it.
Discussion
Hmm. I've had no issues with my profile across 10+ apps since day one. I have one or two paid relays that sometimes don't auto auth'ed so they don't always get posted to. Or a relay may be down temporarily.
One relay going down does not seem to have a large impact on my end, but it can depend on how many relays me and others are on too. You just need to have a few relays in common and you're redundant.
I think the relay hint system has also helped me a few times when I can't see the comment, but not the original post.
If you're having issues, try getting better relays or a better client (Some are a mess).
Nostrudel.ninja has a good tools for seeing which relays notes are coming from.