I don't think OSTree systems can quite reach the flexibility of NixOS. For instance with NixOS (with direnv and nix-shells) you can essentially swap out your running system based on the different directories you enter and I think that's still just scraping the top of the iceberg. From my experience with OSTree (which is admittedly somewhat limited) I don't think you can reach that level of flexibility.

It's still really cool, I don't mean to shit on that, I'm just saying NixOS and OSTree have different pros and cons and use cases.