Reality begins to sink in
It's not HTML5 in principle which is the problem so much as the many disparate implementations of it. Most browsers implement a close approximation of the spec and the outlying specs like web workers, WebGL etc. but there are so many browsers, so many JS engines, so many CSS rule engines, so many differences in the way that things like video, canvas and audio are handled that it must be a nightmare to produce all but the most trivial software with HTML.
This would be especially true on phone / tablet devices where the app must look and feel as close to native as possible under the constraints of HTML imposes. An app would probably have to sit under some middleware like phonegap which provides access to other phone features and use a lot of CSS to get itself to layout properly in every form factor.
Tools like GWT or AJAX libs might be able to help abstract away some of the differences but it would still be a horrible amount of work and at the end of the day it might just be easier and produce a better experience to use another kind of framework.