Good UML diagrams are hard to write, easy to understand.

I used to write a lot of UML diagrams in Mechatronics Engineering for a big company, where specs were not supposed to change often. For projects with a long lifetime and a slow change velocity, UML is totally justified.

UML diagrams are not worth it for Software Engineering: code evolve too fast to keep your diagrams up to date. In that case, I replace UML diagrams by simple sketchs / mockups and simple tables.