Oddly enough of the 3 tanks I've found that Powertechs are the ones I never see.
I've raided with Legacy of Bane, Rage, Infernal Harvest, Terror/Jivanikas, and never seen one Powertech tank (Powertank) for Operations or Nightmare Pilgrim.
Have I seen a Powertank for Gargath and PD? Yes. Have I seen a Powertank for Tier 1 HM FPs? Yes.
Have I seen a Powertank for any Operation (even Story EV) and HM LI? Nope.
Maybe there just weren't many on Ajunta Pall and just aren't many on Jung Ma, but I find it telling that there's so few of them.
I've done HM EC with 2xTanksassins and a Juggernaut+Tanksassin. I haven't done it with 2xJugg but I did Nightmare KP with 2 Juggs and that was faceroll, even in a Columi-Rakata mix.
I'm an Officer in Infernal Harvest (Gloom) and I have one of each tank. I've completed HM TFB with both Annaconda (Powertech) and Gloom (Assassin) and I'll be running on Toxic (Juggernaut) tonight. I have preferences due to playstyles, but all 3 tanks are "Viable".