I am not 100% on this one, but I believe the Unreal 3 engine WAS a completely rewritten engine, as opposed to Unreal 1 to 2.5 being based off the same engine.
The way I look at it, is that the ID Tech engine was originally made for Quake 3, before it was upgraded to create Doom 3 and Quake 4. That is pretty damn old, and I know the reason they don't create a new one is the time scale, but the basics of Radiant are obvious. I know they continue to upgrade it each year to allow a higher FX limit, and larger limitations, but the engine still runs the program, and I don't think poor scripting can be blamed for all of it.
Still, I am not sure exactly the issue, so can only speculate, but having used Radiant a few times, I can see that it is pretty poor. Hell, even companies like EA, Bethesda and Valve have made statements on how trashy the CoD engine is, and they pretty much all stated that the series should take a hiatus while a new engine is put in place.
My knowledge of the engine is novice to amateur, but if these companies are slagging it off, and you would assume they know what they are talking about, then you would think that they are right.