The C4 Engine is a comprehensive suite of professional, robustly implemented game programming tools for PS4, PS3, Windows, Mac, and Linux.
Eric is a very good coder and has made a very nice engine, the code is clean and professional and the engine is as stable as rock. But unfortunately the tools aren't very artist friendly, they don't support multi screen setups, many developers use more than one screen so this is a major pita for fast work, instead of working almost exclusively on a 3D view it requires you to perform some tasks on the 2D orthographic views more times then what is desirable, the window where you pick and search for your models and game objects (called nodes in C4) is a very complicated and strange hierarchy tree like system, it should be much simpler then that, i will not mention other engines but theres much better implementations of this on other engines.
Eric is also a coder that tries to put too much stuff in the same basket, what do i mean by this, for example he tries to make all himself without using any other 3rd party software on his engine, for example the physics engine, Eric instead of implementing a already made physics engine, like the majority of his concurrents, he made his one, it makes for a better integration and support is true, but unfortunately it slows down the development of other important features, and the physics aren't that good, objects don't behave realistically, for example theres no self rotation when an object is flying from a explosion, and sometimes the character controller becomes stuck at the boundaries of floor geometries. Because of this Eric is slow at implementing more visible stuff, things that could make the engine stand out, HDR that is being promised for a looong time now but is being pushed further and further away, the grass system and the underwater capabilities that could make the engine really stand out are also still to be made, etc, Eric prioritizes stuff that don't really give much exposure to his engine and because of that, a very good engine is being brushed aside for engines imo much worse.
I can't say enough good things about this engine. The community is very supportive and active. The engine is in constant development and the quality of the engine code is unsurpassed (for instance I have never seen such short build times for a project of this size).
Coming from Unity the world editor can be a bit intimidating at first but having full access to the engine, rather than the black-box Unity is, is invaluable.
I do recommend reading the two published books about the engine to get started quickly.
The C4 Engine has become one of the most impressive engines out there while still maintaining a low price tag and providing updates free of charge. Not only are new features constantly being added, but the community is one of the most helpful I've ever seen.
One of the key things that makes the C4 Engine stand out is that the sole developer of the engine personally responds to all questions through the forums and provides the highest quality of support possible. Not only is he extremely knowledgeable, but he is dedicated to both the engine and it's licensees and goes out of his way to ensure that your game succeeds.