Selfhosted

I did this. I kept both.

I have Synology photos and immich.

Synology photo is faster to load photos when you scroll to a random date. Also on mobile, the map view is fast and reliable. Also the Android widget can show images of a specific person instead of a random photo.

Immich has better face detection and significantly better search tool. But is super laggy when you're looking for a specific date and time and location.

My Synology is a DS 1821+ with dual nvme cache and 32gb mem My immich is running on a proxmox using a late model Xeon processor and NVME gen 3 and 32gb mem. Nothing else is running on the proxmox.

This is not a question of software (that answer will be yes, because you can then easily switch away from synology once it's becoming bad) but instead a question of ability fo install it on your currently available hardware.
If you are able to utilize docker containers to the satisfaction of immichs guide? You should.

The machine-learning face detection and picture search is amazing.
I have searched for "Shelf with black boxes" in german (the default context model is only english but they have multi language models) and it found the picture of the shelf with my black tea containers.
Or searching "food" brings up pictures of food and dishes, or "moon" of the moon. Honestly I am still amazed :)
(I also like the geo-tagged pictures on a map feature)

I've kept both. Synology photos does auto upload from my+wife's phones. Then Immich scans this as an external database to import.

Synology keeps the photos in a normal name and folder structure. So the files can be accessed and organised in a normal file browser. Moving to a different device or app is as simple as copying the files over. These folders are backed up off site so even if my hardware dies, I don't have to set up Immich again before I can access the images as they're normal image files.

Immich auto upload puts the photos into its own database that is not human readable. I didn't want that. But I do want the AI tagging, search and fast browsing Immich has. If I didn't already have Synology photos set up, then I might have tried using Syncthing to send photos to my NAS and then access through Immich.

If you have all of the photos, then you could use a tool like Immich Go, which would automate much of the tagging and album creation. I don't know if Immich Go accounts for metadata from Synology. However, Synology might be able to export everything.

I would also recommend putting Immich remote access behind Tailscale (I.e., pointing the remove access server to your tailnet and using that domain name and the Immich port to login).

I am a fan of Immich but know nothing about Synology. Does Synology have a subscription or something? Immich has done a much better job at tagging things and putting them in to albums than Apple’s Photos ever did for me, it took the stuff that Apple had tagged and than did a much better job.