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.
By properly isolating the muscle, you will make sure the muscle is strengthened in its full range of motion and as a result the muscle will grow strong, mobile and bulletproof to injuries.
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.
Shoulder Training Mistakes
1. Momentum (Hips or Spine)

Bad form front raise: too much traps and momentum.
Using momentum of your hips & spine is the most common sight you see at the gym when people do lateral or front raises.
Using momentum decreases the activation of the shoulders and skips some crucial parts of the range of motion. Momentum will allow you to use heavier weights but the shoulders aren't effectively trained.
When it comes to isolations, you want the targeted muscle to do all of the heavy lifting. Using momentum does the opposite. Even if you feel a nice pump / burn in the shoulder area, the targeted muscle is still activated poorly.
2. Using Scapula (Shoulder Girdle)

Scapula is intact and only the rear delt is worked.

Scapula moves too much and traps dominate the movement.
One of the most common mistakes is to use your scapula (traps, back muscles) to perform the movement instead of your shoulders.
Front raises are often done well, lateral raises are cheated quite a bit but rear delt raises are rarely done correctly.
In all shoulder isolation exercises (front raise, lateral raise and rear delt raise), you want to stabilize the shoulder girdle (scapula) and only move the shoulder (arm). Otherwise you will barely work the head of the shoulder.
In the video the examples are exaggerated. In reality it doesn't take a big movement of the scapula to start messing up with the quality of the exercise.
You may need to cut the range of motion and focus on slow and controlled movements.
3. Not Isolating the Shoulders At All

Apart from bodybuilders, basic shoulder raises are many times disregarded because shoulders get plenty of work done in workouts when you are doing push ups, pull ups, bench press, shoulder press or movement training.
Just because the shoulders are "worked", it doesn't mean the shoulders are fully activated and they are worked well in their full range of motion.
Lack of proper mobility training of the shoulders can result in tight / painful shoulders and hinder their growth.
Adding a little bit of rotational shoulder work (internal & external rotation) and doing some basic shoulder movements, can help a lot in terms of strength, size and mobility.