Roughly 25% of the time the extension fails to load. Meaning if you aren't paying careful attention as the page is loading you could get BLASTED with loud audio that you would assume you could attenuate with the mouse wheel... until you try it only to realize that the extension didn't load and so you proceed to get blasted with loud audio until you turn down the volume some other way.
There will be times you would be watching something in 240/360/480p and sometimes you assume that it's just the best quality available until you finally realize that the extension didn't load and Youtube just decided to play it for you in the aforementioned terrible resolution.
Lastly I do NOT care about "What's new in Enhancer for YouTube" at all. And the fact that there is no way at all to disable the whats new thing from popping up is pretty frustrating.