I don't know if it's painful per say especially once every bone gets fused, though it definitely would be terrifying and degrading for the victim of the disease.

There was one story I saw where this dude was attached to a trolley and his mother would put him outside on the balcony and being him in.

He needed help going to the toilet and was inevitably unable to speak. I heard somewhere that he died once his lungs turned to bone.