HTML5 Stencyl builds (with some changes and workarounds in engine) is good actually, working better than flash, but, HTML5 itself has many minuses.
- no distribution
- game is not saving on the computer after downloading, so, player got to load it again if the game was closed (very big minus)
- no distribution
It is very hard to make popular HTML5 game, while it is not require any plugins players prefer flash for small games and unity for big 3D games.
But stencyl itself needs many improvements in current functions.