Re: How dangerous . . .
I voted in the 'somewhat dangerous' category because I think if uke is giving you a strong attack, then there always exists the 'possibility' of some injury. If uke doesn't have good ukemi skills and nage is not 'taking care' of uke, then there exists a possibility that uke could be injured in the fall/pin. If uke is giving a strong attack with intention to do damage and nage is having an off night or not blending correctly, there might be a good WHACK forthcoming. I think I would rather get somewhat bopped with an honest attack on the mat then take a hit for real and be completely surprised.
While I think a certain amount of soreness, bruises, etc. is going to result from serious martial training, that doesn't mean that you can't create an mutual environment of safety. For the practice to have integrity, however, I believe it's important to go out to the edge from time to time, which, in my mind, does lend itself to the 'somewhat dangerous' category. This is not to say that I approve of folks that just beat on each other, but the nice place about the dojo is that, hopefully, you've built up a level of trust with your training partners so that you can really 'bring it' from time to time and walk that edge.