The problems with length are not around bytes vs. characters, but that length counts code points. Many logical characters are composed from multiple code points, and some logical characters have multiple representations in Unicode.

For example, consider “á” (U+00E1 latin small letter a with acute). The same logical character could be composed of two codepoints: “á:” (U+0061 latin small letter a, U+0301 combining acute accent). So while they produce the same visual output (the same grapheme), the strings containing these would have different lengths.

So when dealing with Unicode text, it's important to think about which length you need: byte count, codepoint count, or count of graphemes (visual characters), or the actual width (there are various characters that are not one column wide – the tab, unprintable characters, and double-width characters e.g. from far-eastern scripts come to mind). The script in a previous reply takes these different counts into account.

The issue of multiple encodings for one logical character should also be kept in mind when comparing strings (testing for equality, matching, …). In general, you should normalize the input (usually the fully composed form for output, and the fully decomposed form for internal use) before trying to determine whether two strings match.