Ironclad network layout without a lot of flexibility.
Does well with pre-processed artwork but falls down pretty hard on dynamic assets.
UnrealScript is only of use with the Unreal Engine, so your knowledge doesn't travel with you to another engine.
A somewhat arcane series of config files and build chain.(Not horrible given the complexity of the engine but still witchcraft until you learn it.)
Unity built in networking is pretty bad, however there are a few free or nearly free alternatives that will just plug and play.
Unity GUI is a resource hog and you'll be looking for an alternative if you push to a mobile device.
Unity is stuck on DX9 for the foreseeable future due to MAC compatibility issues(essentially they don't want two feature sets between windows and MAC)
The short answer is to try both of them. I prototyped the same game in both engines and found I was 3 to 5 times more prolific in Unity using C# but for the art that could be pre-processed UDK generated a prettier render.
When I tried to "color outside the lines" it felt like UDK fought me a lot harder where with Unity you can beat it into shape pretty easily if you have a decent mastery level of the scripting language.
It's a bit off-putting to see people get so teary eyed over a software change when we're ostensibly supposed to be software engineers, but then again I guess that may be part of the why rather than in spite of it.
Just to go in to pure kiss-ass mode for a moment, for a decade or more GDNet has been THE community to learn about game development as well as providing a great community of like minded people to kick around non-game development ideas. Taking your ball and going home over some CSS and infrastructure changes seems a bit silly.
Dave, Kevin, Frizzlefry, Gaiden and company have provided a great user experience for thousands of folks for a long time. Hundreds or maybe thousands of you have gone from "why does my naked void pointer give me a memory leak" to producing images of the day and full blown games. Take a deep breath and try to not be that same pain in the ass customer you so despise dealing with in your 9 to 5.
Good software is iterative. So remember this roll out the next time you're throwing a tantrum over whatever ultimately replaces this one.
As for the single line of ad text there's only one important detail to recall.