Kindle, the reading device with by far the largest market share, is basically the IE6 of ereaders - too big to ignore, and at the same time dragging down the entire ebook ecosystem with its crappy renderer. Amazon has shown little interest in improving it for over a decade now, while simultaneously fragmenting its own ecosystem with a variety of different proprietary formats that support different CSS and features.
ADE, while less common in new devices, is still very common in much older devices - B&N's eink Nooks were based on ADE at least as late as a few years ago. (Perhaps they still are?) ADE is closer to IE5 in terms of CSS support!
At Standard Ebooks we're often hamstrung in our attempts to make beautiful ebooks by these big players refusing to improve their renderers. We're forced to dumb down our CSS and use outdated techniques (like occasionally having to use tables for layout!) because ebook renderers are so bad.
iBooks is the top tier renderer, because as far as I can tell it's basically a wrapper for an up-to-date Webkit; next is Kobo - also Webkit-based - along with other Webkit-based indie apps. The rest of the big players are far, far, far distant.