They can cause all sorts of problems (like running outside can as well obviosuly) but because you have a relitively small area to run in you put extra pressure on your joins to keep blanced and usually shorten your stride for fear of hitting the front of the treadmill. Having a short stride is the worst technique to have. Also, because the treadmills is moving backwards it can cause a 'breaking' effect when you run before momentarily pushing your foot away. This can also put extra stress on your joints, especially the knees and doesn't harden you up like road running does. If you train for a race on a treadmill then run your race on the road you're likely to hurt yourself.
The best type of running is off road track or footpath. The ground is softer so not as likely to get impact injury from the roads.
I'd say 80% of my running is now off road. I haven't ran a treadmill for about 5 years.
Shit, that's bad. Thats me off the treadmill now
