I think people need to be less scared of the source, we are in the business of making games, this does not mean we should be limited by what is provided, that is the beauty of what Epic has given us, unlimited potential. What if you were working on a game that had a feature where you wanted volumetric lighting? Would you just go “Okay UE4 can’t do it, so I wont try”.
I can tell you right now, the game I am working on, would not at all be possible without source access, I have already made, I would say, hundreds of changes to the engine to accommodate my vision. I am glad that Epic provided us with the source code, and I would not want it any other way. This is what has stopped me from using any other engine so far.
And just to clarify, this has nothing to do with fixing the engine, none of the changes I have made to the engine were to fix a bug, it was to add a feature that I wanted for my game.
Also if you want to see the full list of developers who have contributed to the engine in one way or another, just open the credits list in the UE4 editor, there are quite a few.