The main thing to me was that since this runs on Nextcloud its more extensible as the photos are just stored under the files and you can use various other apps to do what your heart desires. The other aspect is you get your own Gdrive alternative. You may or may not want this.
For mobile compatibility Nextcloud is better since you can choose which folder photos go to and you can essentially automatically backup albums whereas with Immich you can't automatically specify which album photos from a directory should go [1].
In addition to this, Immich isn't too stable yet and each time you update the server all clients have to be on the latest version, at least since the last time I used Immich.