Maintaining documentation. UML cannot go even one developer-week without being updated, but the developer will often not do that. By the time your UML is a month out of date it is useless at best, and misleading at worst.
This has nothing to do with UML. Ad-hoc diagrams or plain text descriptions become outdated at the same speed, which depends only on the level of detail that you put in documentation and not on the format if it. If you cannot keep up with the changes in the code, you are choosing the wrong level of detail, that’s it.
Ad hoc diagrams are much more likely to be tossed in the trash can after a week. If you keep them longer than the same problems apply. If you only keep them for a week than ad hoc is good enough as everyone still remembers what the symbols mean from when you created it, and the next time it won't matter that the symbols mean something different
