Cheaper/smaller lenses have more pronounced brightness fall-offs towards the outside edges. When you stitch these become dark bands where the photos overlap, and they're difficult to edit out.

Yep. In the reviews at www.dpreview.com , this brightess falloff is called "Vignetting", and will vary from camera to camera. I can see it very clearly in some of the panoramas I've taken, not so much in others. Depends on the lighting of the subject area.