My guess would be lack of actuators. For instance, this robot looks like it has an ankle that can only go up and down, but not roll like a human's. Also, I wonder if there's a center of gravity issue, as it almost always appears to be leaning backwards to even out.

I think it's still pretty impressive in its recoveries, even though there's an unnaturally large number of them necessary. About 8 seconds into the video on the homepage, it almost misses and ends up slipping off the second step. I've eaten shit at missing a couple inch curb, though I don't think "graceful" has ever been used as a descriptor for me. So the fact that it just recovers and keeps going without issue is impressive to me.