This is kind of an interesting concept. I'm sure other people have already discussed this, but I suppose it could work to use nostr relays as your personal cloud.
It seems like it is an app into which you can log on any device....but I think that means you'd need to enter your nostr private key into the device, which isn't something I'd want to do. Perhaps I'm misunderstanding the concept, though.
You’re right to be cautious — entering your Nostr private key on multiple devices defeats the point of self-sovereign identity. A better approach might be using a client with delegated signing or NIP‑46 remote signing. That way your key never leaves your trusted device.