Con HTML5 is very dependent on browser performance
Theoretically all browsers should run HTML5 pretty much the same way, but thats not always the case.
Something that worked fine on Chrome, for instance, might malfunction on Firefox (or vice versa). And there's nothing the devs of Construct 2 can really do about it, but to hope next Firefox update might fix it. Internet Explorer is not even recommended.
Add to that the fact that exporting to mobile or desktops rely on these sort of stripped down versions of web browsers (Node webkit, Crosswalk, Ejecta) that you pack with your game, and you can have a real headache if you're trying to make your game work properly through multiple platforms.