So, first off, lemme bring up your gripe with the Rek'Sai mini-rework;
For example, why did Rek'Sai have to get an untargetable ability? The champion design was solid, so instead of buffing her global map pressure or something she was meant to have, they gutted her global pressure entirely and replaced it with an untargetable ult.
Riot stated pretty clearly that the direction they were taking with Rek'Sai was to move power OUT of global map pressure, and into something that felt thematically appropriate (an execute), to bridge the gap between high tier play and low tier play. Also, it's not like you could target Rek'Sai anyway during either form of Void Rush, since pre-rework she moved too fast towards a tunnel to target, and post-rework she dives underground.
And on top of that, altogether, they were buffing something she was meant to have - Kill pressure. They did that in not only in damage buffs, but reworking her Void Rush to be a powerful execute tool.
Now, onto the subject of Untargetability in general;
There are 7 Champions (by my count) who have some modicum of untargetability; Camille, Ekko, Xayah, Kayn, Zed, Rek'Sai, and Fizz. Each of their untargetability skills have some degree of counterplay (some more than others), and all also come at some cost.
- Cost? It's her Ultimate. Counterplay? She can be knocked/pulled out of it to end it early.
- Cost? It's his Ultimate. Counterplay? You can make him waste it on healing himself and deny him damage.
- Cost? Mana and a modest cooldown, as well as a defensive ability. Counterplay? You can move away from it to decrease the AOE and damage. You can also make him burn it as a defensive ability.
Kayn - Cost? It's his Ultimate, and he needs to damage a target to use it. Counterplay? You can take him away into a dangerous location or set up an ambush when he Trespasses you.
- Cost? It's her Ultimate, and she needs to damage a target to use it. Counterplay? You can escape from Rek'Sai's leap (though it requires good timing and some form of mobility), and it's an execute, so it doesn't work quite as well if you're at high health.
- Cost? It's her Ultimate, and her only real defensive ability. Counterplay? You can make her blow it as a defensive ability, and avoid the damage from her feathers.
- Cost? It's his Ultimate. Counterplay? You can stun/lock down Zed immediately after, and move away from him to deny his damage.
In short; chill, dude. It's seven Champions out of one-hundred and thirty-six. Learn to play against them and it won't be a problem.