I suppose it works and it's a neat little addon, but there are problems that are quite annoying.
1. When watching YouTube videos in fullscreen, bottom right corner with the channel name gets focused for a split-second over any video, with a fixed interval.
2. Hovering over items for preview on Steam Workshop for example causes previews to disappear until you re-engage with them with your mouse manually (hover away and then on again) with the same timed interval as with YouTube.

I understand those are limitations of a kind, but I believe potential users should be aware of the stuff I described, and who knows how many potential issues MouseVader could cause on all sorts of websites (also a 3-star is more likely to bring attention to this, no hate here)