[hsivonen]

Treating grapheme clusters as fundamental is slightly problematic in the sense that then the fundamentals change as Unicode adds more combining characters. A reasonable programming environment should still provide iteration by grapheme cluster as a library feature whose exact behavior is expected to change over time as the library tracks new Unicode versions.

Depending on the task at hand, iterating by UTF-8 byte or by code point can make sense, too. And the definition of these is frozen regardless of Unicode version, which makes these safer candidates for "fundamental" operations. There is no right unit of iteration for all tasks.