a medicine ball is a great tool for creating instability to challenge core muscles in a different way. most of the time, people have problems with stability and endurance rather than strength, especially when it comes to the core. if you have a shoulder problem, it is not a good idea to do a plank, especially on a medicine ball since they might aggravate the injury more, also there are other exercises that can work on your core that do not put any impact on the shoulders.