Also, ferrets play roughly with their mouths a lot - if you want them to be more gentle, they must be trained to do so. Ferrets will play with any object within reach, whether or not you intend it to become their toy. They love squishy toys, but when they swallow them, the very least that can result is an expensive vet bill. Vet bills for ferrets in general are expensive - higher than those for a cat or dog. It may be necessary to crawl around and look for any openings that they can get through.
Some furniture can be dangerous to ferrets, too.
Sliding glass doors can also be dangerous, as a fast and sneaky ferret might follow you out one and get caught when you slam it shut behind you. Ferrets with their tendency to play roughly do not do well with small children. If you have a small children, you may wish to reconsider purchasing a ferret. Litter training can also be a problem. Ferrets can be taught to use a litter box but they have a tendency to just go if one is not readily available.