Just a small note about it: it's not open-source and their excuse is… pretty poor. They don't give you modification rights at all, so you can't even legally contribute to the project.
The platform support is implemented inside "plugins" and they are under AGPL-3.0, so… can you even distribute the application, considering that the licence of the application and plugins seem to be incompatible at my non-lawyer first glance?
Their excuse for their application licence so that they can legally prevent people from uploading ad-infested versions in Google Play and similar platforms under their name… but that's why MPL 2.0 and Apache 2.0 have trademark exclusion clauses.
I tried that, and wanted to like it but the lack of video recommendations killed it for me. I couldn't even see videos from my subscriptions. Major disappointment.
I use it as my main YouTube app on my phone, it's working nice.
One drawback for me is the failure if you have too many YouTube subscriptions (>200-300). I removed some creators and it's working fine. You see Grayjay's likes and comments on videos but there are very few of them (you also see YouTube comments and likes). I wish the community around it grew, it has potential.
