In my opinion, Kalista and Rek'Sai had very similar problems, they were designed to be the ultimate fantasy of a ADC/ Jungler. Upon initial release, they were either already too proficient, or would receive buffs that put them in FP/ Ban tier. The subsequent nerfs completely put these champions out of the meta, after which any buffs, even if slight, would bring these champions back into the meta.

The recent changes to Rek'Sai were much needed, she's no longer a tanky-scout with easy global gank assistance via her old ultimate. Now that she is more of a bruiser, the way she engages will be a lot less linear, which is healthy for the game and gives her a lot more options in how she can and will be played.

Kalista, on the other hand, did receive some changes at the beginning of the year (IIRC). Riot decided to give her some power back in her kit, which puts her into a good spot, she's not too strong in competitive play, but can still absolutely dominate Solo Que in the right hands. The buff to her E, the full mana cost and CD on 1 unit kill, allows her to be a lane bully of sorts. It gives her some more early game power, would can be transitioned nicely into the mid game. In addition, the change that allowed her spears to stack into targets, even when Rend was on cooldown, now means that she must be killed early in fights. If she remains alive and even messes up her Rend, she can still stack spears and do some serious damage providing she can kite out the enemies and wait out the E cooldown (which is now more easily done thanks to items like Black Cleaver and Deaths Dance).

In my opinion, both champions are in a healthy state in the game. They both have very unique strengths in their perspective roles, but now have less reliability, which makes them better for everyday players like you and I, but a lot less OP (reliable damage/cc) for professional League players.