Is myositis ossificans pretty rare?

Yes and no: Myositis ossificans progressiva is a very rare genetically inherited condition. Myositis ossificans which occurs in injured muscle or around surgical sites is not as rare but not common either.

Yes, MO is rare: Myositis ossificans occurs rarely in areas that have sustained significant soft tissue damage to muscle. The injured muscle can appear to "turn to bone" as the muscle calcifies, hardens and possibly limit function and motion..