Seems incredibly risky. If a muscle is stimulated too greatly without the user's ability to feel it cramp or be overworked, it could potentially rip, rupture, etc.. and now the user will be dealing with rhabdomyolysis without even having any notice.
True risk. My partner is a paraplegic and their are things to watch out for all the time. Hot items on paralyzed skin, bed sores, muscle atrophy. I don't know if this is common, but my partner has developed a heightened sense of awareness for many of these things. Not to say that muscle tears couldn't happen, but there are other senses that do kick in with practice and awareness to keep people safe.
I'm sure folks who get this treatment will be willing to stick to activities less strenuous than an intense CrossFit workout.
You don't need to do crossfit to get injured. You're perfectly capable of injury by pushing your arm back, walking too slowly, and many other "normal" movements if you ignore the pain or are not able to feel it. This is basically a one-way interface and as far as I understand it, the patients don't get the sensations coming back.