Work hardening is a little like physical therapy. The idea is to simulate work activity to see if you can do it and transition back to work.
That sounds great but in practice, some work hardening facilities don't know your real job duties (they just get a job classification from a generic list), and some think they are assessing psychological factors like "pain or illness behavior" or "symptom magnification".
These are insurance buzz words for "you are faking or malingering" - and the people making these "findings" are only physical therapists - they can help you stretch, use proper body mechanics and give tips on overcoming soreness but they're not doctors.
The key is to always be on guard - these facilities get almost all of their patients from workers comp insurance companies and sometimes "evidence" that someone is somehow not giving full effort (according to a physical therapist) is more valuable to them than getting you back to work.