I have seen friends have a lot of bruising problems when on prednisone - so I would expect that's the culprit.
I don't notice a weak neck but I have 2 herniated discs in my neck and in a lot of pain - my fear is the MG has weakened my neck causing the disc to herniate - so not sure I feel comfortable having them fixed as maybe just more will do the same - however the pain is at a level I cannot tolerate either - geez! And then there is the risk of going under anesthesia also - what a wonderful disease to have!