From my perspective, each app has been successful in their own way - they appeal to different people for different reasons.

Revanced is just a modded YouTube - mainly appealing to people looking for YouTube premium features without the subscription.

LibreTube is the closest to Revanced, however focuses more on privacy by using a separate per-instance login system to sync user data across devices, and proxying videos through Piped instances. The proxy instances do get overloaded quite frequently as of recent, this can usually be solved by changing the instance in the settings.

NewPipe (SponsorBlock fork) takes things a bit further, with no login system whatsoever, no use of proxies, and using a custom extractor, but brings support for other streaming services such as SoundCloud, Bandcamp and some other video sites. You can do crazy things like create playlists with tracks from YouTube, Soundcloud and Bandcamp seamlessly, and listen to these in the background, without needing to provide a single login.

Grayjay appears to focus on following the creators themselves completely independent of the underlying platform, from a single app - removing the need for separate apps for each. It supports a lot of sites so far - the only ones relevant for me being Patreon, Nebula and SoundCloud. It is actually very time consuming to keep up with creators supported via these sites, without regularly opening each of the separate apps and visiting profiles directly - now they can all be under a single roof, in a single app. It seems most comparable to NewPipe so far, but you do need to log in to each site. IMO it’s going to appeal mainly to people who want to actually support creators outside of the YouTube platform.