I've used shoulder holsters of most types and for guns of all types and sizes. Some work OK, and others are worthless.
I find them to be the least concealable of any type of holster, and one of the easiest to spot on someone else. You really need to address how the holster fits and what your wearing when you have it on, and what you do when you do have it on. What may look good to you in the mirror, may be a nightmare the second you walk away and do anything different than your pose in the mirror. A simple shrug or reach can print the straps across your back, or a slight lean forward will let the gun swing out of you coat if it isnt closed, among a number of other things.
As was mentioned, they can be hot, and they can be very uncomfortable, especially if you have it on all day long. They are even worse if they are not fitted right.
My biggest peeve with them is, what I leave the house with, has to stay on the whole time I'm out. With an IWB holster, I can take my coat off, even if I'm not wearing something under it to cover the gun, and still get away with it in a lot of situations, like driving in the car or sitting at a restaurant. Even if I do wear something under my coat, I can still easily access the gun with a normal draw. You cant do this with most shoulder holsters.
My favorite shoulder holsters so far, and the one that gets the most use, is the Galco "Executive". Its made for guns like the PPK, SIG P230/232, S&W J frames. Its a very minimal holster that has not retention straps on the gun and other than the soft shoulder loop, just a strap that goes across your back and clips to your belt. Its one of the fastest shoulder holsters I've used, and it can be accessed from a couple of different positions and draws, like from up underneath, or down through the top. I usually wear mine under half zip sweatshirt. It sits very flat against your body, and has minimal forward swing.
Looks like this....
Even though its my favorite, it still is only used for certain uses, mostly in the winter, and always as a back up to the P229 in my IWB.
When it comes to concealibility goes, there is NOTHING better at concealing a medium to full size pistol than a good IWB holster, nothing.