When comparing Genome2D vs UDK, the Slant community recommends Genome2D for most people. In the question“What are the best 2D game engines?” Genome2D is ranked 47th while UDK is ranked 73rd. The most important reason people chose Genome2D is:
It's the fastest gpu-based framework out there for flash. It's beautifully optimised. It has very low rendering latency, low level OpenGL calls that other tech simply cannot do (ie Unity) due to Stage3D, and thus can render a lot more data quicker
Specs
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Pros

Pro Lightning fast
It's the fastest gpu-based framework out there for flash. It's beautifully optimised. It has very low rendering latency, low level OpenGL calls that other tech simply cannot do (ie Unity) due to Stage3D, and thus can render a lot more data quicker
Pro Cross-platform mobile, desktop and web
Supports Windows, Mac OS X, Linux, iOS, Android, Web and native Flash.
And with the HTML5 export, it also potentially supports development for the Wii U :)
Pro Haxe!
Haxe is a strictly typed programming language that saves development time but still compiles high performance executables, and can build for tons of different platforms (flash, c++, html5, java, c#, etc.)
Pro Access to direct draw features
Has access to direct draw features so you can make you own rendering structures (scene graphs etc).
Pro Automatic dynamic batching
Automatically batch geometries with dynamic batching techniques (by using constant buffers).
Pro Component based architecture
Pro Large toolset
UDK has an extremely large toolset that allows creating almost anything without having to use a 3rd party tool or plugin.
Pro Triple A track record
UDK is an engine used by many big name companies for popular games such as the Gears of War series.
Cons
Con Lacks documentation
The API documentation is minimal, there's not many tutorials and the ones that are there are very small and only cover the basics. If you want to learn how to properly use it, you have to ask the community or read the source code and figure it out.
Con Not too many games to showcase it
Con Slow development rate
Con Superseded by Unreal Engine 4
Con DevKit only runs on Windows
Even though UDK can deploy games to run on multiple platforms, including Mac, it does not feature support for development on Mac.
Con Limits games to FPS
UDK was originally developed for the FPS, "Unreal Tournament", and it certainly shows. While UDK is a very powerful engine, it takes quite a bit of work to make it do something other than an FPS.
