The Alpha1 and Alpha3 releases, along with turning off MPEG-DASH is still working for me on all videos. The Github page no longer hosts those files though. I managed to save them and it's a good thing I did. They work at 720p and you can't FF too far or time search a video without the video dropping out, but for basic HD playback, it's work just fine. I can wait until they release a better version.
Alpha2 and Alpha4 don't work at all for me and for most people that tried them.
As they say they work till they don’t work.
This problem needs a lot more investigation. I firmly believe Google/YouTube have finally called time on apps/addons that bypass their ability to display advertisements from their clients so unless I’m mistaken a more permanent fix is some way off yet.