muntdefems: I also use NewPipe. It's great, although sometimes video downloads fail too. But anyway I don't see it as a true substitute for the original Youtube app: you can only search and play individual videos, but doesn't allow to manage your account (e.g. looking for new videos in your subscripted channels, adding them to your "watch later" list, etc)
I agree if you're someone who doesn't use account specific features and doesn't like recommendations then NewPipe can be a replacement for Youtube app but otherwise you'd still need it.
Nirth: The speed could be better but otherwise it worked for me. IIt's using .m4a as format, maybe that was an issue? I also noticed how clean it is to not have those annoying recommendations thrown at you. Support for background playing if you you are only going to listen to music is useful too.
I was able to download them but I wasn't able to play them, I have AIMP on my android and it can generally play m4a's (which I tested).