Doku is great with Phasma as leader. he is a spam attack machine esp vs Jedi and his stun
however GS is versatile for all combat scenarios, very good health (too much imo for a DPS) and most importantly easier to farm for shards. Savage Opress has high HP, but his attacks aren't great, I classify him more as a support tank with a finisher than a true DPS
tbh I think GS needs a HP nerf. Doku needs a nerf on how his bonus attack procs on top of other procs (ie counters and Phasma proc) in exchange buff his health a tiny bit