They may be able to get close BUT they would be crippled using UDK. You need source code access instead of only a scripting language so you can do advanced things :) . Last i checked this cost 500k for unreal :) This is why a lot of developers extend on Ogre/os engines instead of using udk/unity where you are going to feel…
It was your choice of words when saying "Just a scripting language", What you can and cannot do is entirely up to what is exposed to the developer, not weither it is a scripting language or not. Which in this case something is not exposed in UDK and they would need the sourcecode, yes. Unity for example is very expansive…