Seafile is extremely fast. I don’t use syncthing so cant compare.
I use SSO with authentik and works very well with multiple users.
Android app doesn’t have 2 way sync if you need that (nextcloud doesn’t either).
There are no limitations with self hosted as far as I know. I just wanted a simple sync all that works with sso for multiple users and it works very well.
For services that I want exposed, I use traefik reverse proxy (jellyfin etc).
For things that warrant an extra layer of security I use wireguard and then also traefik reverse proxy with HTTPS but it’s only accessible locally (vault warden).