Nextcloud is built for generic files, so you'll not get all the "smart optimizations" for images. Immich is intelligent enough to serve you a decent enough compressed image at first, and load the full image in background while you start to stare at it. On the other hand, Nextcloud will show you a loading banner.

Immich also does use CLIP to cluster similar faces and so on. Not only that, geotagging, camera details etc.