We've recently added HTML component support for Android https://github.com/Jasonette/JASONETTE-Android/pull/43 and in this case we're using a full WebView.
The reason we didn't use a full UIWebView for implementing HTML initially was because I thought it would degrade the performance a lot.
We are still not sure if this is the case for iOS or not since we haven't tried, but we at least know that for Android it works smoothly.
This is not the top priority issue for now since the top priority for this project is to support all the important native features first and supporting HTML5 is not our priority. But feel free to open an issue on github about this ("Do a spike on HTML component using UIWebView") if you're interested. I personally won't be able to get to it immediately but there are other people who may be interested in contributing.
Hope this helps!