Vanguard tanks have it the worst since so much of their mitigation comes from shielding, which can be negated by crits. Guardians on the other hand can really benefit from War Leader gear because they rely more on defense. A well-geared and specced Guardian can be laughably difficult to bring down. That's why you see them in ranked (plus their mobility for huttball).
Meanwhile, if you're a Vanguard, they just want you to go Assault. If you're a Shadow, they want you to go hybrid with dps gear because that is the OP variety. I'm not aware of too many pure shadow tanks in PvP.
This is a 10-4. Guards/Juggs are beasts because saber attacks can be defended.