TL;DR: In order to work with UDK fast you need UDK system specific knowledge and working with it akes more to modding. In Unity you just need a goal and the docs, this is closer to actual development. Depending on your knowledge of each tech,one will seem ridiculous compared to the other (this goes both ways). If you know UDk inside and out, having to create systems from the ground up will seem as wasting time. Since you know what to modify, in order to avoid the coding/compiling routine, why not? If you use Unity a lot, forced to work with someone else's systems will look intrusive and requiring "magic" skills. Since you know how to code something, why modify something similar, it sounds un-optimized and not-clean.