This is 100% untrue blaming-shifting nonsense.
My statement is based on three things:
1) webglstats.com measures a wide range of devices and produces an overall support number of 96% (notwithstanding a minor recent bump). This matches data I've seen from our own measurements, and also matches what is implied from browser vendors talking about it.
2) The architecture of WebGL is basically a thin layer on top of OpenGL calls. So it works the same as a native engine. Once the calls reach hardware the GPU performance is identical. I am strictly talking about GPU performance here; if you have a game which is slow due to CPU performance, that is unrelated to what I am stating here. To work out if your game is CPU or GPU bound, you will need to make measurements and consider the data carefully, which I've not seen any evidence of many people doing before they start complaining about what they have assumed the problem is.
3) Many, many times I have been shouted down by someone who claims HTML5 is slow and doesn't believe this technical matter of poor quality drivers and GPU blacklisting, sometimes even accusing me of being deliberately deceptive. Then I ask for their game or some kind of evidence of why it's slow. Then I investigate. Then it turns out it was GPU blacklisting all along. If you are in the unlucky 4% it sucks, but that's not a fair representation of the wider hardware ecosystem.
Again, the irony is it's not HTML5 that is immature - it's the graphics drivers, which are native technology. As I repeatedly have to say to people who don't seem to believe this, graphics drivers are widely accepted to be terrible and can even completely ruin game launches. Here's two links to back this up:
Batman: Arkham Knight had such poor PC performance it virtually ruined the launch - it ran fine on console, where the drivers are good quality and predictable
"How to delay your indie game" specifically calls out struggling with GPU drivers: "Despite claiming to, their drivers just don’t properly implement the OpenGL specifications... Then a new OSX update will hit and everything will change again, yet at the same time I will need to still cater for the previous releases and older machines... it’s unbearable... Would I support Mac again? No."
In technology, knowing who to really blame is a surprisingly difficult and nuanced problem. It's rarely as obvious as it seems. For example lots of people, even engineers, blame the browser for bloated web pages.
I am so tired of this that unless you produce any evidence to the contrary, I will just ignore any further complaints. I would actually be super interested to see any evidence to the contrary, because I never do. And if you don't provide any, I would kindly ask that you take a less accusatory approach to the issue.