HTML5 YouTube plays just fine on my newish (2 years old) laptop, which has a discreet Nvidia GPU, with Chrome (no matter if I'm on Windows or Linux). However, the temperature of the GPU increases noticeably while watching videos (i.e. it is doing more work) and the speed of the fan increases, too. It's like this with every element I put to play. With Flash, any increase in resource consumption is unnoticeable.
Some days ago I booted my old (5-6 years old) desktop, which only has Intel integrated graphics, with Chrome. In HTML5 YouTube, playback skipped a lot of video frames, and often sound would have interruptions too (especially if I switched tabs or windows and went to do something else). The CPU became much busier. This would happen on both Windows and Linux. With Flash, videos play just fine, and the CPU definitely doesn't get as busy. The version of Chrome is the same on all four OS installs.
Conclusion: while I like HTML5 video much more than Flash video (if not for anything else, because I don't need plugins, even if they are bundled with the browser), it still has a long way to go, as currently it uses much more resources than Flash to achieve the same goal. I believe this is the main reason why YouTube still makes Flash the default: if they made HTML5 video the default, it would play poorly for a lot of people, and make their computers super slow.