Kael an endgame poisoner? No way. He is not even in the same league as Occult Brawler or Frozen Banshee.
Kael's A3 can apply up to 4 strong poisons and his A1 can apply a small poison. I agree...Frozen Banshee is great...that's why I have her 6 starred and on my CB team along with Kael. Everyone else on my CB team is strictly there to keep them two alive.
You are underestimating Kael.