The screenshots are not impressive at all. But I saw a great project made with it. It is powerful and can look extremely pretty. They use some interesting solutions to create semi-realistic dynamic lighting (e.g. via cubic light maps). This approach is simply faster than real dynamic lighting.

The renderer is fast even with parallax lighting. The engine is optimized to work with those shaders so it is simply the fastest affordable out there. No chance to achieve the same with another tool.

I talked with experienced coders used to work with different engines. They agreed that you can prototype with tools like Unity3d or Gamestudio but if you want to create something really working with big levels, good scene-management and visuals then C4 is simply superior.

Especially shader geometry needs good clipping / culling / scene management. Otherwise your graphic card will suffer from the amount of polygons, textures and shader batches.