Honestly, I have been disconnected from the development side of nostr for a long enough time that I don't have a good feel or opinion about data handling to know how I feel about health data on a private relay. I have minimal exposure to how blossom works.

As for data standards and hardware... All I will say is that people are SUPER picky about what they will wear. I spent years trying to find a pebble replacement that I liked and settled on Garmin despite a lack of an open ecosystem. I think you would be hard pressed to get significant adoption if the platform can't take in data from other health ecosystems and hardware