Biggest problem is that it costs just as much as a Prom-S bolt to fire(1 full energy unit per shot). Second, IMO, is the shot velocity that only plays well with the HL-7s and the Prom-R. Third(debatable) is the weak fire rate(lower than the Prom-S).
Fix at least two of the above(and while you're at it, maybe make the Prom-R not useless), and I think it might be worthwhile.
The steep increase in energy usage from the S-Breaker to the Circe is easily the most jarring, though. 5x the energy consumption per shot, 250m less range, and slower fire rate, in exchange for +50mps shot velocity and 5 more shield damage per shot?
EDIT: Although, the increased lethality and ammo capacity of secondaries compared to FS1 might be at least part of the out-of-universe reasoning.