LibreTube - uses Piped as video source by default. Subscriptions and playlists can be created, all without actually interacting with YouTube.
Clipious - uses Invidious as the video source. Also allows for subscribing and accounts. Lesser known client
NewPipe - Gets the source directly from YT. Allows for subscribing and creating playlists
Additionally, there’s also ReVanced that lets you patch the regular YT app to include useful features.
I think the issue is that the more users these instances have, the easier it is for them to get throttled/banned by the services they want to use. I self host piped and it works 99% of the times
Also some services are frequently changing stuff or just making it impossible to have an alternative frontends