There are not many Sith with uniques so typically you zeta a leader. Then that zeta is lost in a Sith team if that character is not the leader, i.e., choose between zMaul or zVader. This is definitely a downer as they can be great pieces. The more successful Sith share labels with Empire, I believe. And one of the most powerful Sith doesn't have a zeta, what??? (EP)
Otherwise for arena, they can be useful in a hybrid team, i.e., different factions (empire, etc.). My arena is team is that (zVader (lead...yeah who does that? lol), zCLS, zR2, Thrawn, Tarkin). I used to have EP in there but notice it's Rebel/Empire. I would like more love for the Sith and if EP gets a zeta...
They do make a big impact in many areas of the game:
Fleet (great ships) - TFP, DV, Maul, FOTP (capital) -Chimaera, Executrix
Rancor (zVader teams)
HAAT p3
TB/TW
as I mention above, blend for arena
Sadly they are not doing well as all Sith in arena.