A couple of important missing features is the ability to directly iterate utf16 strings codepoints and appending codepoints to existing utf16 encoded strings. For iterating codepoints, one implementation can be found in ICU documentation[1][2].

[1] https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/utf16_8h.html#a844bb48486904fdca40c8b883e9c80ee
[2] https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/utf16_8h.html#ae98a64ae0f42bc6ad4179293c3638be4