Having been around ultralights for almost 40 years now, I think the crash and fatality data is closer to dirt bikes than road bikes.
Yes, like with dirt bikes, their are fatalities flying ultralights. But even so, the vast majority of the "accidents" I'm aware of in ultralights hurt the pilot's pride and the machine more than actually hurt the pilot. And the vast majority of the accidents never make it to the papers.
There is not a lot of mass and a whole lot of "crumple zone" in an ultralight structure. A vertical crash and the quick stop that entails is going to cause a lot of injury or possible a fatality. That's true.
However, most ultralight "accidents" are not of that variety. They are horizontal decelerations because of blown takeoff, blown landing, off field landing, etc. Bending aluminum absorbs a lot of the force in most of the "accidents".
The other thing ultralights have is that there isn't enough mass or speed in an ultralight for the deceleration to be very violent. [I heard some stats years ago on the difference in running off the end of the runway in a Cessna 150 right at rotation vs the average ultralight. The mass of a Cessna 150 decelerating generally will badly damage it and hurt the pilot badly. The mass of the ultralight decelerating generally only bends it a bit and the pilot isn't injured.]
If one gets some training and flies with a proper level of care and caution, I don't believe flying an ultralight is more dangerous than riding a motorcycle.
If one either doesn't get any training or is an adrenaline junkie that constantly pushes the envelope of their skill level, then I think an ultralight will be more dangerous than a motorcycle because of the unforgiving nature of flying in general. You can probably push the envelope too far repeatedly on a motorcycle and get away with it. Not so much with an aircraft [ultralight or other].
My thoughts.