Short wheelbase & narrow track = inherit instability. Greatly increased weight vs. a motorcycle of the same displacement means more energy transfer in a rollover resulting in crushing trauma.
Quads are more dangerous than motorcycles in pilot-induced errors, although the impression is they must be safer. The "ATV's are safer" meme is a myth.
The American College of Surgeons did a study of 53,000 motorcycle and ATV accidents and found that you are more than 50% more likely to die in an ATV crash, and 50% more likely to need ICU admission if you survived. This was not solely related to helmet use.
And, yes, the pilot must be properly licensed, and the vehicle must be properly tagged.