Well, it has a pretty bad AF compare to other cameras around this time. It is pretty much the same AF system that was in the 5D. And if you ever tried to shoot in more darker scenarios the AF can be pretty much frustrating.
I must have a good one...
I've used my 5D for night shooting and not had a problem. If later Canon cameras have the same focus system they may lag other manufacturers but that in itself doesn't make them bad... and certainly I've not had any great issues.
If I knew how to post an example night shot, I would