I'm convinced that Haunt is the worst main ability as of right now. The best argument I heard against it, and forgive me but I can't seem to find the original poster, was that it promotes a "lone wolf" style of play in a team based game. If someone kills you, they appear with the thermal ink effect for a little while and only for you, encouraging you to seek them out and kill them back. You can maybe justify its use in a team based game, but even then there's better uses for your limited slots.
My major objection to it is that even when it works, it doesn't work for you:
If someone kills me, it's usually a charger, a jet/heavy, or a stealthy roller/sploosh/dapple. The first three weapons are long range and couldn't care less if people know where they are. Everyone knows where they are anyway, almost at all times. As for the stealthy weapons, having the thermal ink effect applied to them doesn't matter because they're probably sharking around and won't show up; they only show up if they're standing up and behind terrain of some kind. Off the top of my head the only weapons that Haunt would work well against is the squiffer or goo tuber, because they tend to be standing up to charge, they can be in a variety of places (usually behind scenery charging), and their range is not justifiable for camping.
Haunt used to be a good ability, providing uses for backline weapons. If I was playing defence and got killed by a flanker, my whole team was alerted. Now it just promotes aggression that'll ultimately get you killed again.
As for stackable abilities, probably cold blooded. It's just hard to justify its use with so few meta weapons having point sensors and with Echolocator being a thing of the past.