I only got as far as page 12 on phone so forgive me if this is answered, but
The entirety of the developer preview is subject to change, and that includes IE11. When 8.1 is formally released, we will be downloading that formal release. Is it beta or not? Does it matter? Arguing semantics is just foolish. Fact of the matter is we are using a "preview" of 8.1, and it may very well see bug fixes to any given component thereof in the final release of 8.1. After 8.1 is released I doubt IE11 will receive further updates until the next OS update. So there will be a final release, and what we are using may or may not be it.
Next up- popups. Most porn sites. Really the only places where the popups will be a bother on a phone, but I get mixed results. Some open in the same tab, others in a new tab.
I haven't used Android for a while, but I remember hating all the browsers I could find as recently as mid 2013. None of them rendered text sharply, it was all fuzzy, they all handled reflow stupidly, and the UI design is usually nav controls covering the an unfortunate portion of the screen or full screen mode where you have limited nav control so you need to swap back and forth. Never a happy medium. They were stable, but I much preferred the crash happy IE on WM6 because while somewhat slow, it was just designed better. The older opera mobile 9(?) on WM6 was too slightly unstable but designed and functioned with intelligence.
IE11 on WP8.1 has a similar layout to WM6 IE, you have all functions available in minimalist nav controls, reflow was replaced with some zoom thing that works well most of the time, text is sharp, and crashes are rare. From where I'm standing, IE has offered a consistently good core experience, while the Android browsers have fancy bells and whistles but are lacking in core experience. I personally have no need for bells and whistles- IE11 gave me more than I ever wanted in a mobile browser. I just wish I could easily push pages to and from my phone, then I can die happy.