To get strong, mobile and healthy shoulders, they need to be trained properly. Here are the common mistakes to avoid.
When you want to isolate a muscle, you want to isolate it as much as it is humanly possible. This means decreasing the involvement of muscles you don't want to target.
Yet, a lot of people do isolation exercises like they are big multi-joint compound exercises: they use their whole body to move the weights instead of solely using the muscle they want to target. This creates poor results.
Isolative work for size or mobility should be done with slow tempo, control, mind-muscle connection, light weights (just enough to feel it) and high repetitions (10-20+).
Isolations and mobility exercises shouldn't be considered strength work and the goal is not to lift the most weights possible. The goal is to fully develop the muscle and you can only do this with the correct form and execution.
When you want to build strength or power, you can use your whole body to generate force. For this purpose movement training, big compound exercises and plyometric drills are excellent.