The fact that it doesn’t provide any benefit or make things particularly simpler than describing them in code and projecting them to UML if you need a visual diagram. Also, the fact that it is particularly counterproductive if I’m not coding in a pure class-based OOP language, since it presupposes that the solution is centered on class-based OOP.

There a diagrams that are useful before coding, but most of them aren’t part of UML, which is a product of the simultaneous highwater of class-based-OOP-is-everything and pre-Agile industrial software development methodologies, and (except for some isolated bits) poorly fit to anything else.