The only problem I've seen is that the difficulties are too easy. Expert is as hard as it should be, If you get stuck with shoddy players, it will take a while to get through. On the flip side, normal is waaaay too easy. I can get through it by myself (3 npcs) without a scratch. I imagine the npcs could get through easy on their own if that was possible.
Expert is really fun though. The hunters, boomers, and smokers are annoying, especially when you are the last one left. Once, I threw a molotov, got caught by a smoker, dragged through the fire, and then a hunter jumped on me while in the fire. Needless to say, I did not survive.
The tanks are nearly impossible on expert, but the only reason that you ever get killed by a witch is if there is a stupid player in the server (Valve knows their audience well).
Also, I figured out a near-foolproof way to survive a horde rush. Stand in a corner or another place with only one entrance, and hold down right click.
(My Steam user name is either ildrean or George Bulwharks, depending on whether or not I'm playing in a group with the other Bulwharks)