Using both the floor and the bed allows for increased support for your neck and thoracic spine. Performing those exercises on a ball challenges the neck and upper thoracic spine and is therefore not appropriate for people with significant acute pain along those areas. A patient should at least be able to hold their neck in a neutral position to avoid ...
"Evidence of healing of partial-thickness rotator cuff tears following arthroscopic augmentation with a collagen implant: a 2-year MRI follow-up"
Source - https://www.ncbi.nlm.nih.gov/pubmed/27331028