Neoaxis is using C#. my main problem is that huge code package which should be used and extended with own classes, I simply can't see through it. probably requires more c# experience. and yes, its documentation is very limited. it's okay if one of the examples covers your game system, you have to tweak it only. its model importing workflow also seems to be not the best.

on the other hand, Esenthel engine (free or 100usd for a commercial project) offers you C++ or its own scripting language lying between C++ and C#. its documentation is also limited, little text and a well commented header browser of engine classes, but there are a lot of small tutorials those can be used for step by step learning, and also as little bricks of your code. its model importing is cool with many formats, visual physX body creation for models fast and easy, you can define and inherit object classes and properties in the world editor easily, its terrain system is perfect for huge worlds (consists of streamed areas), offers navmesh pathfinding, very nice shadows and effects in deferred rendering mode which is fast in DX10+ mode, nice lod with billboards, and fast gpu bone animations, for 5 usd you can have access to make custom shaders, and also you can buy some source codes and game sources (a good quality fps source is free). but it lacks static and dynamic batching/instancing which is cool in unity. you need to put always a terrain in your level, as objects are handled as part of an area, but e.g. somebody is working on a spherical chunked terrain system in Esenthel, so for good programmers it is a nice choice.

I'm also trying UDK, but had little progress in it. I do not care its editor too much, rather its scripting, which simiarly to Neoaxis has a lot of stuff you don't really know what for, and unrealscript is said to be abandoned in next engine release I mean UE4.