I was required to use UDK as part of my University course and two words I would not use to describe it are "very stable". In fact, at one point I was using three computers to work on three different areas simultaneously, because the time taken for it to close and then reopen after a crash was longer than the time it spent working before it crashed. In writing up the process, I provided a selection of "in-development" screenshots, all of which were the "UDK.exe has stopped responding" dialogue box overlaying some area of the level.
Granted, this was between 2010 and 2012 so things may have changed since then. It's definitely not something I want to relive, and I've actively turned down jobs that require use of Unreal Engine and UDK.