Having gert big bits of metal swung at you is dangerous I am sure 
Not if you're clad in heavy armor, using blunt weapons (with regard of the safety of your competitor).
And yes, jousting, even during training, would be highly dangerous. Numerous sources tell of people dieing in such tournaments (with blunt lances), mostly from falling of their horse (as said above)
I've even read a case where one knight got wounded in the saddle, causing all others present to start fighting, ending in the death of several spectators
