Right now only Light Disaster has the unique Mode Gauge size of basically 50% her HP being a full gauge. Fire and Dark have a fairly large Mode Gauge aswell, but can still get into Stun 2 times during one fight.

But, the major thing to keep in mind is, that I specifically said "after a certain point in power" she is viable. To clarify this more: Sieg specifically needs help to make use of her full potential, which is not an option if it already takes you 10 turns to get something into raging. Same goes for the Punch Dummy. If you dont have a proper setup, you gonna fall short, rarly overshoot though. The extra hit from her Assault Relic helps her keeping better control over Rage Gauges. The extra Buff from her Hammer helps out for regular usage to provide more party wide offense.

Siegfried is not a Soul that works by herself as the likes of Arthur or Herc. She needs more thought in her usage aswell as some support. She is by no means something for a beginner to pick up, I'd even say she isnt something a semi-veteran should pick up, but for veterans (meaning, proper understanding and knowledge of the games mechanics) and high-power caliber players, she might be exactly what they want to make more out of what they already have.

Public raids are often enough a problem due to either having the lack of information about the bosses status (bug that doesnt update normal-rage-stun properly) or to many people that it lags heavily (or the fight is just outright over before anything major has been done).