The only real problem Source has is that you have to license the engine to get full access to all source code and get rid of the limitations that the Mod SDK imposes on you.
Cryengine and Unreal Development Kit doesn’t have this issue.
And then if you have access to the full source code you can make any adjustments you care to make. Improving the engine.
Making a game engine takes a lot of work and time so it makes sense to reuse the old code and incrementally build on it. Most game engines are improved this way even if they slap a new number on it.
So it really isn’t about which engine is the best but it is about who can make the most use of it.