I quite like Swift's approach -Characters, where a character can be "An extended grapheme cluster ... a sequence of one or more Unicode scalars that (when combined) produce a single human-readable character.". This seems, in practice to mean things like multibyte entries, modified entries, end up as a single entry.

As the trade-off, directly indexing into strings is... Either not possible or discouraged, and often relies on an opaque(?) indexing class.

The main weirdness I have encountered so far is that the Regex functions operate only on the old, objective-c method of indexing, so a little swizzling is required to handle things properly.