[happytoexplain]

>what looks like one character is actually a special case joiner between two adjacent codepoints

Are you referring to a grouping not covered by the definition of grapheme clusters (which I am only passingly familiar with)? If so, then I don't think it's any more non-meaningful to reverse it than to reverse an English string. The result is gibberish to humans either way - it sounds more like you're saying that there is no universally "meaningful to humans" way to reverse some text in potentially any language, which is true regardless of what encoding or written language you're using. I was thinking of it more from the programmer side - i.e. that Unicode provides ways to reverse strings that are more "meaningful" (as opposed to arbitrary) than e.g. just reversing code points.