I think you had bad experiences,
I have seen many organization implemented RUP successfully,
although I have seen much more failed to use it correctly. as I said earlier the biggest pitfall is there are lots of people (project managers) who don't know know technology X (RUP) tried to use it, just because it was buzz word, and they failed.
I have worked with more than 20 companies started using RUP in Iran and among them only 2 project manager knew the different between UML and RUP. so how can one expect these companies to succeed?

first of all the essence of RUP is against the Iranians culture because it promotes promotes discipline and hard work

and also don't forget failing is easier than succeeding, there is an statistics that in US from each 5 new company only one succeed and 4 fail, so if you have seen only 1 out 5 project succeeded using the method X (RUP, XP, SCRUM,...) it is usual, above that is good and below shows a bad result. (and also put the fact that we are in Iran and we will have more problems)

Among those companies that failed using RUP almost all of them never applied iterative incremental aspects of RUP, they defined long inception and elaboration phases and tried to make most of the project in one phase just like water fall and SDLC.