I haven't noticed EPUB suffers from bad support. I like its principle (just a markup language), the two problems I came to see: 1) handling images, tables... objects that may need to be cut are cut at the expense of the ease of reading. And 2) no handwritten annotations.

But you're right: deep down, both depend on the implementation, not on the format itself so you could actually blame the support. (Yes, I somehow changed my mind in the process of writing this comment.)