It’s not clear to me why reversing a string is dangerous or pointless.
Codepoints can combine more than once, to the point where if you're too nitpicky you can't validly substring either, you can only read a string from the first codepoint onwards; they could become invalid sequences if reversed, possibly?
":medium light skintone: :Man facepalming: lol"
(with the lol reversed). In this problem, it is a much harder problem than, say in python, mystring[::-1]. Therefore, it is a different problem "reverse a string" than to "reverse an array".
Accented characters would be kept as is in my scenario.
Unfortunately just reversing by grapheme clusters doesn't solve the problem because of directional formatting codes; if you have e.g. a right-to-left embedding followed by a pop directional formatting you can't naively reverse them.
Although I think grapheme clusters are a pretty good approximation in that it's usually what you want to backspace in a word processor.
1/ Treating a string as an array of bytes will give an invalid result if the string unless the string is simple ascii (or an equivalent encoding where each byte has a clearly defined standalone meaning); in particular, just reversing a UTF-8 string in this way will give an invalid answer - ie a string that isn't even valid UTF-8.
2/ The fix for (1) is to convert your string into an array of Unicode code points and reverse that … except that is also broken, because combining characters will now not associate correctly, as per other answers in this thread.
Coding your way out of problem (2) in a robust and sensible way is, I suggest, a significant challenge.
[1] - https://github.com/microsoft/referencesource/blob/master/Mic...
Suffice it to say, there are many interesting uses for string reversal :)