I think there should be a restriction in the standard on how many Unicode scalar values a graphical unit can have.

Last time I checked (a couple of years ago admittedly) there was no such restriction in the standard. There was however a recommendation to restrict a graphical unit to 128 bytes for "streaming applications".

Bringing this or at least a limit on the scalar units into the standard would make implementation and processing so much easier without restricting sensible applications.