To start, consider that the term 'character' used in the article, though 'generally correct' ... is definitely not correct in the broadest sense.
Western, Cyrillic and Asian scripts boil down to 'characters' with some complexity maybe with ligatures ('Straße'), but it falls apart quickly for other languages.
Unfortunately, rather than creating rigorously applied definitions for things, and applying them consistently, even Unicode falls into this bureaucratic trap of vagaries with their own definitions.
So Unicode works well for most things, but then it falls off a cliff.
Here is the definitions section [1]
Even have a look at the definitions of 'Character' and 'Grapheme' and 'Grapheme Cluster' - and you start to see how confusion sets in very quickly.
Consider that in Unicode ... there isn't really such a thing as a 'character' - it's just an unspecific word we use that has no technical application! (When we say 'character' generally what we mean is 'Grapheme Cluster').
Language is itself a rabbit hole of complexity, so any standard trying to manage it will be painful - but it feels as though the true corner cases of Unicode are actually unbounded.
In short, too many pragmatic loose ends. Given any scenario where you think you have an alg sorted out ... and probably there are holes in it if you cared to try to find them for a specific language.
It's not 'bad', but it's not the uber solution, it's frayed at the edges.