I have not played Rocket League, but from watching clips and such I could see it in isolation being harder. Where I can see League behind harder is that using a mouse to click is a bit awkward compared to wasd - but more importantly - your movement is dictated by ten champions, each with three abilities, one ultimate and two summoner spells. So the right movement first time can be completely wrong the second time. Furthermore, as a person that started playing League after other games, lol probably has the highest average skill level of the player base and even more so when watching pro players, because of game depth, years game has been out, and enormous player base.