I did a bunch of MTBing last year on a rigid SS.
Most common serious MTB injury seems to be busted ribs, from falling onto rocks or similar. The speeds are much lower so falling off is usually not such a big deal.
In MTBing you can buy speed, it's worse then Fatty Masters and TTing. The cost of equipment is enormous and it breaks a lot. The amount some of my acquaintances spend is more than any roadie I know. This puts me off.
With the advent of dual-suspension, the course are becoming more technical, because the average rider can now handle what only experts could safely handle a few years ago. This is bad for rigid SSers.
Most MTBers regard anything other than singletrack as sacrilege. I don't get this. Fitness isn't tested enough.
MTB racing is lonely.
MTBers are very suspicious of anyone who rides road, and are just as closed-minded as any other group.
I still ride my MTB now and then. Cornering hasn't improved much but my descending is way better.