At the very least their app store, which is pretty much required for OIDC, most 2FA methods, and some other features, stops working at 500 users. AFAIK you can still manually install addons, it's just the integration that's gone, though I'm not 100% sure. Same with their notification push service (which is apparently closed source?[0]), which wouldn't be as much of an issue if there were proper docs on how to stand up your own instance of that.

IIRC they also display a banner on the login screen to all users advertising the enterprise license, and start emailing enterprise ads to all admin users.

Their "fair use policy"[1] also includes some "and more" wording.

[0] https://github.com/nextcloud/notifications/issues/82

[1] https://nextcloud.com/fairusepolicy/