Exercises affect people differently because of activation patterns. When going into a squat for example most people will usually virtually only their quads. Why is this? Because they're in a seated position most of the day and their hip flexors and quads are shortened. This puts the glutes and hamstrings in a lengthened position. The result? You're not squatting properly. Same thing happens with other exercises. Lots of people have pecs that overpower the upper back muscles. This is not a good thing. You also have a lot of lifters who want to be doing rows, but turn it into some sort of biceps exercise. Exercises affects people differently, but that's only because they're not using the muscles they should be using to lift.
Now as to whether or not you're one of those people.. I can't tell. I've never seen you lift, I don't know what your physique looks like.
A serious risk of hurting the shoulder doing flies, V? Well, there's a huge risk hurting your shoulder when bench pressing. I agree, if you want to build your chest, the bench press is your go to exercise, but it has a huge potential for messing up your shoulders. A much bigger potential than flies.