I saw something about Arabic text, where that naive truncation at codepoint boundaries turns one word into a different word! Like the sequence of codepoints generate something that is represented as a single glyph in fonts, but truncated its totally different glyphs. I don't remember more details, I don't know any Arabic, but grapheme clusters aren't just about adding diacritics to latin characters. In other languages it all might work quite differently. So truncating at word boundaries (at breakable white-space or punctuation) is probably best. Though of course that way you might truncate the string by a lot. shrug-emoji

(I don't think the talk where the stuff about Arabic was mentioned was Plain Text by Dylan Beattie, but I haven't re-watched it to confirm. So maybe it is. Can't remember the name of any other talk about the subject right now.)