Can't control but they are generally predictable. I tend to ride the same trail loop, so know it's condition based on recent weather.

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.