GameBryo is simply the best and easiest to use engine for huge open worlds like Morrowind/Oblivion/Fallout 3.

Witcher uses a rewritten Aurora engine but this engine is... crap, to say the least. Witcher was CPU based too and not optimized for multicore so it ran very badly on my PC even though I had an excellent GPU.

Witcher was an excellent game, but far from a technical masterpiece..