Yes, it is possible to injure yourself with these machines.  I feel it important to learn as much as you can before you start to use one.  But if used with discipline, it can be helpful for men whose ED is associated with vascular insufficiency.

The machines are "Class 2" medical devices.  This lumps them together with powered wheelchairs, pregnancy testing kits among other things.  There is risk if they are used improperly.  Education may be required to use the device safely or to achieve the intended outcome.  "Class 3" devices are things like heart pacemakers that are inherently risky.

The closest comparison to shock waves I can think of is this:  Wrap your penis with cloth. Place a towel on a table and have your penis rest on it. Now tap your penis with a rubber mallet.  If you tap it gently there is no harm.  But if you hit it hard enough, bad things will happen.

What happens if you tap firmly - but not to the point of any pain or injury - 2,000 times?  (I don't suggest trying this!) Shockwaves provide a tap, but not a physical tap. I have to say it feels physical.  The theory is that this stimulates angiogenesis - growth of blood vessels - leading to better blood flow.

I've had a machine for almost a year.  I've done a total of seven rounds of treatment, mostly following a protocol of 6 treatments per round, 2 sessions per week. With my second round I was too aggressive, and progress was set back.  Since then it has been an upward trajectory.  There is clear improvement, and no harm.

As we get older we can no longer get away with mediocre health habits.  One way this shows up is with ED.  So using shockwave therapy in conjunction with improved diet, exercise, sleep and limiting stress as much as possible is likely to improve results.

Good luck!

None of the studies I've read reported adverse effects from Low intensity shock waves.  To learn more about shock waves and ED, the link below is a good place to begin.  At the bottom of the article are links to other studies.
https://www.sciencedirect.com/science/article/pii/S205005211930099X?via%3Dihub

There are different types of machines, different technologies for creating shock waves, and even differences in what is considered a shock wave.  My opinion: don't sweat any of it.
 

Success rate here?  It's hard to tell.  My guess is more than half of us, but not by a lot.  In studies the success rate has been around 60 - 70% if I recall correctly.  The studies tend to cherry pick by limiting inclusion criteria so they select guys most likely to benefit.

Basically most of the studies exclude guys like these:  If you have had ED for a long time, you are less likely to benefit.  If you have diabetes you are less likely to benefit.  If the source of your ED is something other than blood flow-related, you are less likely to benefit.  If you are morbidly obese, you are less likely to benefit.

For guys with profiles like those above, the success rate is very low.

From what I can tell, guys most likely to benefit resemble my profile: Willing to work on factors under their control such as mild hypertension, a few extra pounds, work-related or other stress.  I've cleaned up lifestyle related issues, and with the SW therapy made significant improvements.