we have 2 main problem in html5
-issues with preloader that not opening game after game was downloaded as long as html5 consists from many files in if some file didn't downloaded the game will not start. very big issue in fact. sites rate game by play time, if people will fail to download the game it can spoil rating a lot.
- no orientation lock on mobile. stencyl games will perform bad when user disable autorotation and hold his device on the different orientation than your game.
additional problem, stencyl games are very heavy for mobile. only very light games only in chrome and only modern powerfull devices can play stencyl games now.
but, stencyl html5 is ok overal for now
here is as example http://ru.y8.com/games/famous_fashion_designer
it is showing good results and was on top of y8