The UI of paper books is better in most ways. Ebooks don’t need separate large print editions, and have full text search. Basically every other point goes to paper books. I don’t bother to defend the aesthetics of books, because their actual utility is high, too.
They’re damn bulky, though, especially when there’s an alternative that weighs nothing. Damn bulky.
I tend to disagree, or at least argue that UI/UX is strongly subjective. I have sought out digital copies of books that I have in paper form just because I strongly prefer reading on an ereader for text. Obviously, something with graphics is likely to be better in paper.
You can't lose your place easily. Lighting isn't an issue if you buy a backlit model. Reading lying on your back or side is much easier. Traveling is easier with an e-reader. Access to wikipedia and the dictionary on the same device.
There are emotional reasons that I like paper books, but if I'm just trying to read, give me an ebook.