I'm one of the main tanks for a 10/10 NMM 16man guild. Assassin tanks are just fine, and when you do the math our mitigation is similar across the board compared to PT and juggs. You will find that we have higher hitpoints to help counter our low DR, and our shield chance/absorb is through the roof. Powertechs have high DR but their defense is the lowest of the three tanks from what I've seen. Also, their cooldowns are terrible compared to the other two tanks.
Burst aggro in the beginning can be a bit rough, but once you throw out your first force lightning from your rotation you will be more than fine.