This is a mix of perceptions and feelings- correct them as necessary. I don't work on the spec nor the implementation of the standard- I speak as a potential user of the functionality. I assert I am still permitted an opinion.
- How to I make HTML Imports production-ready on present-day networks without vulcanize? HTML Imports produces many, many requests.
- How do I reconcile ES6 modules with HTML Imports?
- Will package managers learn to support HTML Import bundles?
- Many, many modern frameworks treat HTML markup as presentation layer, not data/application layer.
This is what I can think of off the top of my head.