alt Hacker NewsUTF-16 is absolutely not easier to work with. The vast majority of bugs I remember having to fix that were directly related to encoding were related to surrogate pairs. I suspect most programs do not handle them correctly because they come up so rarely but the bugs you see are always awful. UTF-8 doesn't have this problem and I think that's enough reason to avoid UTF-16 (though "good enough" compatibility with programs that only understand 8-bit-clean ASCII is an even better practical reason). Byte ordering is also a pernicious problem (with failure modes like "all of my documents are garbled") that UTF-8 also completely avoids.
It is 33% more compact for most (but not all) CJK characters, but that's not the case for all non-English characters. However, one important thing to remember is that most computer-based documents contain large amounts of ASCII text purely because the formats themselves use English text and ASCII punctuation. I suspect that most UTF-8 files with CJK contents are much smaller than UTF-16 files, but I'd be interested in an actual analysis from different file formats.
The size argument (along with a lot of understandable contention around UniHan) is one of the reasons why UTF-8 adoption was slower in Japan and Shift-JIS is not completely dead (though mainly for esoteric historical reasons like the 漢検 test rather than active or intentional usage) but this is quite old history at this point. UTF-8 now makes up 99% of web pages.
I went through a Japanese ePUB novel I happened to have on hand (the Japanese translation of 1984) and 65% of the bytes are ASCII bytes. So in this case UTF-16 would end up resulting in something like 53% more bytes (going by napkin math).
You could argue that because it will be compressed (and UTF-16 wastes a whole NUL byte for all ASCII) that the total file-size for the compressed version would be better (precisely because there are so many wasted bytes) but there are plenty of examples where files aren't compressed and most systems don't have compressed memory so you will pay the cost somewhere.
But in the interest of transparency, a very crude test of the same ePUB yields a 10% smaller file with UTF-16. I think a 10% size penalty (in a very favourable scenario for UTF-16) in exchange for all of the benefits of UTF-8 is more than an acceptable tradeoff, and the incredibly wide proliferation of UTF-8 implies most people seem to agree.