I want to warn everyone about listening to game “journalist” about engines. If they really knew anything about game engines, they would be working in the game industry with six figure salary. Just take what they say with a grain of salt.
I want to give my experience with licensing engines. This was a very long time ago when I worked in the industry. The company I worked for was trying to decide between Quake and Unreal. We choose Quake. We got a cd-rom fed ex to us with the source code. No documentation. We were told the code is well commented. If we needed help, we could schedule a call with Brian or Carmack for $1000 an hour when they are not busy. We eventually moved to Unreal because they had a dedicated support team ready to take our calls 5 days a week during working hours. Plus they had plenty of documentation. The main reason any company choose a big software package is not just the technology but the support that can be provided. My old boss once said in mocking of Ballmer: SUPPORT SUPPORT SUPPORT.