IIRC, Immich started as a project after Photoprism changed their licensing to restrict several features, and then picked up a ton of momentum very fast. They've pledged they won't ever pay-gate features, which isn't always a positive as those features help sustain a project long term, but they have so much community support and love that I'm not worried about that in this case.

Feature wise I think they are pretty comparable (vs the paid version of Photoprism), and I like the UI of Immich slightly more. Immich also supports singe sign on via OIDC easily, which I rather appreciate so my family doesn't have to remember 10 different passwords.

Regarding stability it's actually pretty reliable. I've been running it for a long time via Docker in the form of the TrueNAS plugin and have never had any issues, like ever, so I think it being marked as unstable was a bit overly cautious. I think they have also recently moved to a new phase of development that is also going to be even more stable. Even if it does break, all of your media is stored in a nicely organized directory structure on the filesystem so you're not going to lose anything.

> Immich started as a project after Photoprism changed their licensing to restrict several features

It didn't, that's just a timing coincidence. Immich was started because none of the existing projects offered the google photos-like experience.