It's the old "invisible injury" factor, wc does it to anyone with any sort of hard to prove injury, probably even for injuries that can be "seen" & proved by imaging or the usual accepted tests.
Plus you have to be able to convince them that the injury was due to your job or while on the job.

I would think RSD claims are extra difficult because the documentation of symptoms & timeline has to be tied to whatever the original injury was that caused the RSD.. tough one.