Zero Sum Gain
Every other ult in the game (besides arguably Bard) gives immediate power to the ally team. Whereas Kindred's ult does not discriminate ally or foe; she just gets to choose when it pops. Sure it's impactful, but it's existence on the field works just as much in the enemy's favor as it does yours. And for what it does, there are far more reliable, less risky ults out there. Their ult doesn't make sense given their overarching theme either. At all.