> Indexing & substrings are common, too.

Indexing code points in both UTF-8 and UTF-16 requires reading the whole string up to index location. Substrings are the same as well.

> Right, and for 1 billion Chinese speaking people UTF16 is 2 bytes/character, UTF8 is 3 bytes/character.

That's true for a text file without markup. But most text is not like that in 2017. HTML is probably the most common text format nowadays.

So let's see how a popular Chinese language website does.

  curl http://language.chinadaily.com.cn/ --silent | wc -c
  52678

  curl http://language.chinadaily.com.cn/ --silent | iconv -f utf8 -t utf-16le | wc -c
  93368

So UTF-8 seems to be quite a bit more efficient in this case, 52678 bytes. When converted to UTF-16, same page was 93368 bytes.