The example you used wasn't the best for 'Press R be better', Syndra? All she does is fire some balls at you from a click shot, that's not really better, just damage. But a frustrating mechanic that comes from it is button press abilities.
Basically, things like Aatrox R, Kai'Sa Q and R, Zoe W, stuff like that. Aatrox R used to be just some damage in an area and more range and attack speed but they change it to have his revive on too, meaning it's now just a press R to do more damage and survive, instead of him timing and stacking his passive well skillfully to be better.
Kai'Sa is the worst example. Q is just press button do damage. That should not exist in the game at all, there is 0 skill in that at all, less than Syndra's R. Kai'Sa's R is even more stupid as she doesn't even have to mark her opponent to zip to them, a teammate can. Then she just flies in with the press of a button, then presses Q, right clicks, gets a penta and everyone says how skillful she is. No she isn't, she pressed buttons, she didn't even have to aim.
In all honesty though, 'Why don't you hold still'/Mobility is key, is the most frustrating. Kai'Sa exists because of champs like Yasuo or Irelia, don't have to land a skill shot on those stupid champs if it's not a skillshot. :P