When your life flashes before your eyes, when you find yourself scrambling for a hold while suspended in mid air, when you hit the ground in any number of painful twisting, bouncing, smashing ways, and when you've been dragged as well, from getting caught up in the reins or stirrup, you'll not find 'unplanned dismounts' to be dull at all. Getting trampled is also a very real issue in certain situations. One gal online showed pictures of her helmet after her horse smashed it (yes, it was still on her head- at least initially!). You couldn't have taken a bat and recreated the mashed disaster that was that helmet. The girl didn't even get a concussion, but she'd be very dead had it not been for that helmet and a bit of luck saving the rest of her body.
And I see it's been at least partially covered, but think bones, tendons, joints, and/or nerves. These are all key to keeping your arm functioning happily. You can search for nerve damage in this forum and get lots of 'hits'