Well, I'm not sure what to make of the engine. Most parts of the game look quite stunning, especially the areas with dense grass and foliage covering the ground, but there are some things that are strangely out of place, like something from a 2002 game. For instance, most of the smaller Korean encampments in the outdoor levels have very poor textures inside and almost no lighting to speak of. The textures on many rock cliff faces are another example, and there is often a jarring difference between them and the grass textures leading up to them.
And it does show signs of bad optimization. This is masked by the amazing graphical quality at maximum settings, but it becomes apparent when you turn down settings to low or medium. It look similar to Far Cry with the options turned down (which is still quite good, but nothing like the game on high), but there isn't nearly enough of a performance increase.