A lot of Rek'Sai's kit doesn't make much sense anymore
Why does her bite scale off of fury when you can build it up so fast? There's basically no point where you'd ever have to use it early. Before the rework, it made sense for it to scale because it took much longer to build up to max fury, but now you hit max in an instant. Why not just make it deal max damage all the time? The interaction between her E and her passive is kind of unnecessary now.
Her Q used to be a way for her to build up fury faster by hitting multiple targets. That was a large part of why it did AOE damage. Now that that interaction is gone, it's lost a lot of depth and is an ability that deals barely noticeable damage to surrounding enemies for seemingly no reason. Its aoe portion is useless now, just being used to boost your damage against your main target.
Her current passive: At full fury, burrow for 3 seconds to heal 20-190 health based on level. I understand why it has to be much weaker since you build up fury so fast but 20 health at level 1 is ridiculous. Before, her sustain was much greater but now her passive barely helps her stay healthy in the jungle even though that's its whole point. There's also lane-sai who could abuse the passive if it were any stronger, due to how fast her fury generation is now.
Her tremor sense got nerfed because of pro play, so moving enemies leave a tremor every 1.5 seconds now. That is a pretty large delay in information, especially if you are tracking someone who's just around the middle of your detection radius because they could basically go anywhere and you have to wait that long to find out. This can make chasing people in burrowed form quite tedious and frustrating even though you should have the upper hand. The benefit you get from blinding yourself is 15-30 bonus ms but that simply isn't enough to compensate for that huge loss. You feel like an idiot, chasing after where someone was and trying to predict where they might move only to be lead around for an awkward amount of time before finally catching them or giving up. It'd be better if enemies who were closer to you left behind tremors slightly more frequently, like maybe if they were at your inner half of detection radius it would be a 1 second interval. There could be problems with that idea but it's just a thought.
Last of all her tunnels. Aside from being used as a dash, leaving tunnels around the map was meant to open up locations that you could teleport to with her old ultimate. You were rewarded for creating tunnels in useful locations all over the map. Now that she has her new ultimate, which i do like, the interaction there is also killed. Before, putting tunnels all over the map gave you power in a way. Now there really is no point in doing that. They just make neat little shortcuts.
https://i.imgur.com/8Msdny3.png
TLDR: Rek'Sai lost a lot of depth as a champion after her rework with the removal of her old ultimate and the ability to build up fury so fast. The resource has become so easily attainable that her Q's minimal aoe damage and the scaling on her E are no longer necessary.
The main point of this post isn't at all to say that rek'sai is weak, but to point out things in her kit that simply feel out of place now after she was reworked and her change from being a champion with more depth and a kit that better interacted with itself to a more generic damage dealing champion.
and does slightly increased AoE damage, and slightly decreased target damage to make it feel like it was made for being AoE, not just a comfy AA reset and Sheen proc.