Well, I'm the academic who posted the original tweet. Tweets are not meant to be scholarly articles. They are advertisements, and I think they can also be controversial. Here are my follow-up tweets:
1) UML was created to model OO designs. It effect you are modeling the code of a system, not the system's behavior. UML is at wrong level.
2) the idea that 7 (or 13) diagram formats in UML can cover everything is crazy. What about GUIs, web wireframes, authorization, etc. ???
3) UML has encouraged the idea that models must be graphical. Ridiculous! Text and graphic models are both useful and often interchangeable
4) UML is at once too large and complex and at the same time very limiited. Stereotype and profiles are not effective for usable extensions.
Note that I'm not necessarily saying UML is bad. I'm simply saying that it is not helping the goal of "model-driven development", which is what I'm interested in. I don't understand the comment about "holy grail".