"really hard to balance"
This is a buzz term best comparable to the "it's 2016" argument people try to use to justify their stance on something.
This does not actually do anything to improve a stance with evidence or clearly visible tendencies.
This is a buzz term best comparable to the "it's 2016" argument people try to use to justify their stance on something.
This does not actually do anything to improve a stance with evidence or clearly visible tendencies.
Not really. There are definitely things that are really hard to balance. ie: 
Generally speaking, if an ability has no counterplay that makes it hard to balance. Particularly abilities that are always active no matter what, like Irelia's Ionian Fervor, Morgana's Soul Siphon, and Twisted Fate's Loaded Dice. This kind of stuff is invisible power, and often it's entirely invisible because it's on a champion whose kit is so old they don't even have a buff bar icon to remind people of their always active passive, so it's on the opponent to not only know the kit from the start, but actively remember these stat buffs that they have to worry about every time they interact with someone.
We have examples of perfectly balanced passives that actually have counterplay and do very similar things to this other free power.
Draven's League of Draven is an interactive way of increasing his gold above others, while giving opponents a real chance to deny it. Better than Loaded Dice.
Ahri's Essence Theft is an excellent way of utilizing her abilities to give her health sustain, and it's a flat number that is not based on her post-mitigation damage like spell vamp.
Malzahar's Call of the Void gives him crowd control immunity, but can be poked off.
So yes, always on abilities with no interaction that basically offer free gold worth of bonus stats to champions are hard to balance. Discount it all you want. This kind of thing doesn't belong in the evolving League game, everyone should have windows of power to exploit (that's not a buzz word. That's an actual term with an actual meaning).