Truncating mid-codepoint produces an invalid utf8 sequence, which some decoders will silently ignore or replace with <?> glyphs, and others will fail loudly.

Truncating mid-grapheme-cluster can change semantics in unexpected ways. For example, the "family" emoji(s) can be encoded as a set of conjoined codepoints for each family member: https://stackoverflow.com/questions/49958287/printing-family.... Depending on where you truncate, you might just get the father, which probably won't cause you any serious issues but it might cause confusion depending on context.

(IMHO, if you're truncating a way where this would matter, you should probably be truncating in screen-space, i.e., way higher up the stack)