A motorist plowing me from behind on the highway is something I can't control.
How fast or cautious I ride is in my control. It does help to be familiar with whatever trail you're riding however.
But yeah I agree road cycling is extremely dangerous, you are trusting that 100% percent of the drivers on the road will be mindful of cyclists, all the time.
For new trails, it's helpful to ride with someone who's done it before, otherwise just take it easy.
My cousin had a bad mountain bike accident, and i think it had to do with risk desensitisation. He's an excellent rider, and was just warming up and goofed a bit. Ended up breaking his back and knocking himself out.
I don't necessarily think mtb is safer overall, but the risk factors are more apparent, and, as with any sport, user discretion is advised.