The biggest disadvantage of UTF-16, IMO, is that programmers blindly assume that they can index into the string as if it were an array, and get a code point out

The biggest disadvantage of UTF-16, IMO, is that programmers blindly assume that they can index into the string as if it were an array, and get a code point out — which you can not; you'll get a code unit, which is slightly different, and might not represent a full code point (let alone a full character). UTF-8's very encoding quickly beats this out of anyone who tries, whereas it's easy to eek by in UTF-16. The real problem is that the APIs allow such tomfoolery. (Some have historical excuses, I will grant, but new languages are still made that allow indexing into code units without it being obvious that this is probably not what the coder wants.)