It’s essentially a Safari extension that makes YouTube videos look better, and act like standard HTML videos. Which means they use Safari’s native video player, which means that you have slightly more control over the video, but at the same time you do lose some functionality that YouTube has in their player.
Chris does a nice job laying out the costs and benefits of the extension, and I’ll add that chapters don’t work either, which are a pretty core feature of YouTube these days. What about captions? Probably not, although admittedly I haven’t tried. What about quality? Can I knock down the resolution if my internet is slow? Can I boost it to the highest quality? HDR? I doubt it.
I used to use a browser extension like this many years ago, back when YouTube used Flash to display its videos, but I'm really not sure what we're accomplishing with something like this today. The stock HTML video player is OK, but the custom controls that YouTube allows create a far better experience in my opinion. I know some people have an undine hate for YouTube, and I guess this is going to appeal to them, but for me and I think most people, the YouTube video player is not only fine but pretty excellent.