@Thomas- For the iPhone 5s, you have to disconnect the home button (and fingerprint scanner) from the front display assembly as you open it, but opening it is trivial. The only worry is ensuring you don't snag the home button cable connected to the main board. This isn't super hard but requires some dexterity that makes a DIY repair much harder.
For the S5, it appears you have to heat the front of the display (at least the edges) and gently and rather expertly apply force to lift up the screen assembly. Not only do you have to do that to get the screen out for a screen replacement, you have to do that for ANY internal repair. There is a very high risk of breaking the screen doing this. Given that this is by far the most expensive component, that's a substantial risk if all you want to do is replace the camera/speaker/etc.
My guess is that Samsung decided on this approach to keep it water proof/resistant, but it greatly complicates any repair, ones that would be trivial on other Galaxy devices.