The name of the Ult "Killer Instinct" suggests that the ability should be employed to dash in for the kill, but instead of any damage component on the ultimate, Riot opts to integrate a defensive feature!!!??? Moreover, the shield synergizes poorly with some of Kai'sa's other abilities. To begin with, in team fights, most of the time your enemies are marked with plasma by hard CC, so why need a shield after a dash onto the enemy if the enemy is CCed in the first place? If the enemy does not have plasma marked by hard CC, then Kai'sa has to snipe someone with Void Seeker in order to use her ult, but wait, the range on her W is 3000, whereas it's 1500/2000/2500 for her ult, meaning that she can't follow-up after "seeking out" an enemy who is out of range.
The skill is just weak and underpowered for an ultimate. Compare this to Nocturne's ult, which has a range of 2500/3250/4000, a 1.2 AD ratio damage component, and cannot be interrupted. Oh, and a global blinding effect to boot. Also, there is no requisite condition to use the dash, and Nocturne can press W to get a spell shield upon landing (better Synergy).