Your answers will be massively different from rider to rider, e.g, when I used to ride virtually every day and pushing the limits of my skill to improve i would break many things on a regular basis, work basically funded the trials habit. Sump guards, brake pedals, gear and clutch levers, engine covers, chains, tensioners, discs, handlebars...... You get the picture. The harder you practice, the bigger the crashes.
As mentioned before, it's the new or beginner riders that tend to have nasty crashes, not knowing when to get off, or hang on. The expert riders will come off bigger obstacles, but they jump off when it starts to go wrong and land safely.
The nature of trials requires the bikes to be durable and easy to work on and fix. If your stuck in the middle of Ranoch moor during the ssdt, you've only got the contents of your rucksack to save the day.
Injuries, yes. Many. Broken tib and fib, knocked teeth out, elbow doesn't straighten, knee once had yama printed on it, plenty of gravel rash and the usual bumps, scrapes and bruises.
Do we need to make trials safer? I'm not for the wearing of body, very restricted movement could mean more crashes, how about bubble wrapping every rock or even take them away? If your not prepared to get hurt now and again then get a different hobby.
There will always be accidents in whatever we do, everyone has their own risk and pain threshold. Rider training would help beginners initially, but as their skill grows so do the sections.