For East Asian languages UTF-16 uses 50% less memory, these characters take 3 bytes in UTF-8, but only 2 bytes in UTF-16.

    utf16 decode
      if (unit <= 0xD7FF || unit >= 0xE000)
        /* one unit */;
      else
        /* two units */
    utf-8 decode
      if (unit < 0xE0) {
        if (unit < 0xC0)
          /* one unit */
        else
          /* two units */
      }
      else if (unit < 0xF0)
        /* three units */
      else
        /* four units */

    utf16 encode
      if (value <= 0xFFFF)
        /* one unit */
      else
        /* two units */
    utf8 encode
      if (value <= 0x7FF) {
        if (value <= 0x7F)
          /* one unit */
        else
          /* two units */
      }
      else if (value <= 0xFFFF)
        /* three units */
      else
        /* four units */

Things get even more complex when we are to read an untrusted string. In UTF-8 we have five byte types, invalid bytes, conditionally invalid bytes, and must recognize surrogates (as errors). In UTF-16 all 16-bit units are valid and there are only three unit types: character or surrogate, high or low. I once wrote a decoder for UTF-8/16 and here's my stats:

    UTF-8    : 13 byte types, 13 states, 10 actions
    UTF-16LE :  3 byte types,  4 states,  5 actions

    utf-8 decode
      switch (std::countl_one(unit)) {
        case 0:
          /* one unit */
          break;
        case 2:
          /* two units */
          break;
        case 3:
          /* three units */
          break;
        case 4:
          /* four units */
          break;
        default:
          /* not code point boundary */
          break;
      }