Cass empowering E with other poisons is more of a gimmick than anything. There are so few champions with poison the interaction will rarely come into play. In addition Cass can already poison her own targets very easily with her spammable Q and large W. Lastly, the consequence of being poisoned by another champ means if Cass is within her short range, she will deal more damage. Not instant death.
Yasuo can jump in from almost a full screen away, refresh his shield, gain armor pen, deal damage, and extend the duration of any CC. Not only does this mean instant death for anyone within a screen of Yasuo, all he had to do was point and click R once.
Kai'Sa should hardly be used as a model of balanced gameplay. She was given the ally interaction not because she needed it but so she can make LC$ big plays with her support. She didn't get to be the #1 ADC in pick rate by being fair and balanced.