Slope is mainly a problem for 2-story tractor designs -- the kind that's lower to the ground, not such a big deal.
As others have said, though, *uneven* is a real nuisance and may prevent you from using the tractor on some areas. The problem is not just chickens getting out thru gaps between the tractor frame and the dippy ground -- the big thing is, predators getting IN.
For this purpose I would suggest making an EXTRA wide skirt of 1" welded wire mesh (not chickenwire). By extra wide I mean like 2-2.5' wide. You will want to engineer it so it can be flipped up against the tractor wall when you go to move the tractor, or it will hang up on every little bump and anthill plus you won't be able to get close enough *to* move the tractor
Then, have some big heavy cinderblocks handy. The extra wide skirt will conform to the dips in the ground (better than the tractor itself, anyhow) and you can weight it down into the dips thrugh creative arrangement of cinderblocks.
This is NOT a great arrangement for a tractor moved daily, because of the nuisance factor, but it can help with one you move less often.
I just don't use parts of my yard that are too lumpy or dippy, though.
Good luck,
Pat