None of the regular String methods have been coverted to work with Unicode characters instead of with parts of Unicode characters. Maybe this will be on the roadmap.
UTF8Str aims to be a lightweight way to do a few useful things with String bytes and characters, in a way that respects Unicode, in the smallest amount of code, with the greatest amount of clarity possible. Efficiency is not an aim. Simple, maintainable code that does something useful is the aim. If efficiency happens it's a bonus.