Rek'Sai has more base HP and Armor than most tanks and fighters.

So when looking at Rek'Sai's base stats and looking how people are playing her tank not only because of the Q nerfs but because she is just Tankier than most fighters and tanks out there and this is the problem. Something has to be said when Rek'Sai can be one of the best tanks with mobility, pressure, sustain and CC and one of the best fighters with reliable damage and true damage.

So to show base stats:

Reksai

HP at 18: 2100 - 28th best health in the game

Armor at 18: 92.1 - 12th best Armor in the game

Now if you ask me Rek'Sai should not be beating champions that are designed to being full tank in the HP and Armor department. And this is after the health nerf. I don't think there are anymore Cinderhulk champions still dominating as they were when it was at is prime but Rek'Sai with cinderhulk is still one of the most dominant things out there.