One problem with grapheme clusters as length is that you can't do math with it. In other words, string A (of length 1) concatenated with string B (of length 1)

One problem with grapheme clusters as length is that you can't do math with it. In other words, string A (of length 1) concatenated with string B (of length 1) might result in string C that also has length 1. Meanwhile, concatenating them in the reverse order might make a string D with length 2. This won't matter for most applications but it's something you may easily overlook when assuming grapheme clusters are enough for everything.