What are the possible disadvantages of shoulder or elbow replacement?
A replacement joint can never be as good as a normally-functioning natural joint.
The range of movement will be less than with a healthy natural joint. Some movements, such as reaching above your head, may not be possible, although most people will be able to carry on with normal everyday activities. You should discuss with your surgeon before an operation how much they expect your movement to improve, as each case is slightly different.
Replacement joints will also wear out after a time, but it's likely that they'll last for 10 years.