Let's pray to the Emperor your information are wrong for only heretics are known to use the Unity engine.
Unity doesn't support physic on the gpu and only has a 32 bit version at this point. I don't think the 32 bit part is a game crushing problem as a studio I know said they were going to release a 64 bit version of their game. They are indie so if they do pull it off, Behavior can do it better. The physic on the gpu can be changed if behavior has soruce code and with a game this big your going to want that source code.
Might I ask why that is? As I said it's a very versatile tool that isn't specialised for anything, such as UDK which is primarily made with FPS games in mind.
That's why. Versatile does not equal powerful. Sure a swiz army knife can eventually cut down a tree, but a chainsaw would be a lot more effective. Unity has a reputation for being very versatile but not very robust.
Every time I read about Unity it seems to get more and more efficient so provided they are using it I think it's a good choice. IIRC Pathfinder Online is also planning to use Unity, so that would be two MMO aka huge scale multiplayer teams who decided it's a good fit
