I'm curious to know what aspects of HTML5 you think harm UX and SEO.

I was under the impression that overall HTML5 benefited search engine indexing through defining and encouraging the use of semantic tags and embedded microdata. Plus extending the capabilities of the browser with geolocation, drag-and-drop, extended input types, etc.

If by "HTML5" you mean that more extensive browser capabilities lead to the trend of building fully client-side/single-page applications, maybe I could see something there.