"Video Background Play Fix" and h264ify. I primarily watch YouTube on my phone and it's unusable without these extensions. I've had to downgrade after the automatic update.
Seconded. Good UI, quick fixes whenever Google tweaks some things in the background. Also, the ability for background playback is key to listen to music. and the speed and pitch control sliders area lot of fun when listing to speeches (eg Mickey Mouse JFK).
I don't understand how people are able to use Newpipe regularly. I use it to download videos or listen to music in the background, but even there I encounter the 'Something went wrong' error page about 30% of the time (with the latest updated Newpipe every time). It used to be worse, being a 50-50 chance of getting the error page, but 30% is still too high for me to use it as my everyday Youtube client.
For me, NewPipe got a lot better when I added it's repository to f-droid, rather than using the version in the official f-droid repository.