This is very true indeed. Company mode is the completion framework that rules them all. I have abandoned the older ac-alternatives in favour of this. And it's easy to write new backend source plugins too.
I find WWW::Mechanize::Firefox an interesting alternative. The API is similar to WWW::Mechanize, but it remote controls an instance of Firefox using the MozRepl addon. This means scraping/interacting with pages using JS works fine.