As most of us know Flash sucks. On my laptop with a great GPU but a bad CPU, Flash only cares about the CPU so it lags even on 720p steams. Flash is also almost guaranteed to crash if you have 3 flash frames up at the same time. If I watch stream on my 2nd screen, Flash can't handle it being muted and stay full screen, it minimizes once it looses focus.
HTML5 is way more stable and can handle utilizing both CPU and GPU. Even YouTube has a HTML5 alternative for people without Flash.