I'm xFirefoxYBA , A current Diamond player, I've been playing LoL Since season 9 and peaked d4 in my first season at the game. never played any other MOBA before.
Right now (season 10) I'm moving between mid to high Diamond while playing Evelynn mostly.
I also stream on twitch some of my ranked gameplay at https://twitch.Ttv/FirefoxYBA
feel free to come visit and ask questions!

Evelynn has several pros -
A) She is very good for carrying games 1v9 below mid to high plat.
B) She is very self depended and doesn't always rely on teammates and can take care of herself.
C) Has invisibility which is extremely valuable in Solo queue, where people don't coordinate and you can pick off easy targets.
D) Has THE BEST Smite capability in the late game and can turn games around with elder/baron steals.

A)If you are behind you lose a lot of Evelynn's potential, since you are not capable to one shot your targets properly.
B)Weak Early game - She can get harassed by early game Junglers without the ability to fight back,resulting in having to give up camps and early objectives.
C)Ultimate Depended - Evelynn Needs her ultimate in team fights and during early game skirmishes and it has a pretty long cooldown.

Evelynn has a feature that other champs lack - the invisibility.
If you master the invisibility's full potential and you know all the tiny things about it and things you can abuse with it,
you can guarantee to win a lot more games.
Players JUST DONT KNOW HOW TO PLAY VS HER IN SOLO QUEUE.