People have to realize that the Unreal Engine is very flexible. To say U2 or UW will use the Unreal Engine is putting it lightly. Mainly because, the "Unreal" Engine was made for Unreal.
Although the past, present and future games will use the core Unreal Technology, it's not THE Unreal Engine.... Yes, it's a play on words, but it really does make sense if you think about it.
So, you can safely say it's a heavily modified version of the Unreal Engine (Technology will be the more appropriate word). But it's not the "Unreal" Engine (saying it is based on 80% of the first Unreal game's engine).
If I remember correctly, Tim said the Unreal Engine is like an engine to a car. Although the block (the housing for cylinders) is the same, the other components (the type of cylinders, valves, fuel injections...etc...) are different and can cause the whole engine to be regarded as new techology.
Other game companies will actually create a new engine and called it XXXX2 Engine. They may not have anything to do with XXXX1's engine, other than the name and some components.
So, to put this simply, XXXX2 is a new engine core that uses some minor codes from the old engine. Unreal Technology uses the old core, but adds new code to the engine. But, Tim had great foresight when he created this engine, so that adding new code or enhancements could actually change the core (versus other engines where the enhancements won't do crap to the core).
Not to take away the greatness of other engines, but the Unreal Technology is very flexible and advanced. Yes, there are new engines that are better. But it's the longevity of the engine that makes money. Developers feel more comfortable with engines they worked on before than learning a whole new technology. That is why the Unreal Technology is one of the top selling engines.
If you think about it, you can use this engine to create RTS, FPS, Sports, Driving, RPGs and Sim games. Pretty flexible. I would love to see a UT mod or total conversion Basketball, Football or Baseball game
.