There are a few champions in league of legends that incorporate RNG mechanics into their kit. Generally speaking, they're not fun to play with or against.
RNG is inherently unhealthy for champions because it can create excessive feast or famine depending on how RNG decides to roll. Fiora, for instance, can easily get first blood if the first couple of marks that appear are facing towards her for easy procs. Zyra can be a total terror in lane if her seeds spawn in relevant positions or can be a wimpy mess if her seeds are all clustered uselessly around her tower.
I'd like to see the RNG mechanics removed in favor of more static mechanics that are within the players control. As an example, Gnar's form shift. It's easy to plan around and players know when it's going to happen, what causes it to happen, and are in total control while it's happening. Imagine if, while playing Gnar, when he was transformed into his tank form he took all control from the player and mindlessly attacked anything nearby. This wouldn't be very fun, would it?
This is the problem of RNG. Even beneficial RNG isn't that fun. Twisted Fate, for example, can get up to 6 bonus gold per minion kill. What if the TF you're laning against gets super lucky and gets a ton of free gold right out of the gate and easily out-builds you? That might be fun for him but I'm sure it's not fun for you. I'd like it if all the mechanics of League were clear, easy to understand, and were under direct player control and not just able to be slightly influenced.