The big advantage would be that all that metadata would also be decentralised, i.e. I'd have a backup of all that on my own PC, and I could push it to a differe

The big advantage would be that all that metadata would also be decentralised, i.e. I'd have a backup of all that on my own PC, and I could push it to a different host to automatically transfer everything, whereas e.g. GitLab has had to write custom import scripts for GitHub metadata, and you just have to hope that everything maps cleanly if you use it.