Kindred's Late Game isn't really rewarding...
I do believe that Riot is still looking into Kindred. But the problem is for a champion she is not rewarding for a jungler who's supposed to scale, and suffers an incredibly weak early game against most junglers.
She's basically the Jungler version of Singed. They are both fun to play. Their kits are both unique. And their play-styles are also massively different for their roles Juggernaught/Jungler. But they both don't feel rewarding at all. Singed finally got a rework, and it looks like it's actually rewarding players for his playstyle, and making him scale better.
Meanwhile Kindred has a mini game that can go virtually ignored. Because the bonuses they provide are nearly negligible in most cases. Especially with how hard it is for her to even collect those marks because of enemy knowledge. And hell, Riot found a way to make a mechanic that goes virtually ignored by a majority of her mains! The range boost is nice. The speed boost is nice on her Q. But it's just not important at all for her. Because when you really think about it... the only real contribution they provide to you is range and a little extra damage on your E ability. That's about it.
She scales with items... sure... but it just doesn't feel right or good. She's a jungler, that needs to build ADC items. She won't become a threat till the 25 minute mark if she's fed. Or 35 minutes if shes been farming. And even then, her late game fantasy is basically far outclassed by heroes who are designed to scale like Nasus and Veigar who will get completely out of control if left to their own devices. She doesn't snowball like an assassin, Vayne, or Twitch. And her linear scaling falls flat compared to Jinx and Lucian.
And sure! She scales well with crit! Not really. Only one ability actually benefits with crit chance (Infinity Edge does not modify the damage by the way)... which is basically the threshold when Mounting Dread Crits for 50% more damage turning it into a tiny ult with eight marks. And even then, a kindred player would use this ability to finish someone off. Her damage goes up with crit... but others simply do it better.
To duel with her, you need to be SIGNIFICANTLY better than your opponent because their kit is far more mature than the mess that Riot left behind. And her damage... again falls short of what other champions can do at all stages of the game.
And her marks? Getting them is SLOOOW. You can get 15 marks in 40 minutes assuming you were on the ball about getting them immediately.
This is not saying that she's weak. I've got a pretty high win rate on the champion personally. She's fun. But she doesn't feel relevant.