1: Self atk+ (+125%)/guaranteed triple/spread 1t [6]
2-A: Against Stunned: All atk+ (? +?%)/normal attack cap (+?%)/ability cap (+?%)/burst damage cap (+?%) 2t [6]
2-B: All mode gauge reduction+ (+?%)/normal attack cap (+15%)/ability cap (+15%)/burst damage cap (+15%) 2t [6]
2-C: All crit+ (+20% damage, 100% chance)/def- (-?%) 5t [8] (def down cannot be cleansed, only blocked. Also, there is only one crit frame and for overwriting purposes the game considers only the damage portion, so if you for example use inferior Frigg's crit+ with higher damage but significantly less chance, this buff will miss)
3-A: 15x stun nuke + stun+ [8] (damage happens before stun+ is applied)
3-B: All gauge down (-30%), all burst gain (+3) 3t [6]
3-C: Normal attack [4] (use with abi1!)
Passive1: Against Raging: All combo+ (low?)
Passive2: Against Stunned: All atk+ (+?%)
Soul weapon burst effect: Echo burst depending on enemy state (damage is: stun > raging > normal)
Offensive Soul weapon skill: All guaranteed combo against Stunned
Defensive Soul weapon skill: Against Raging: Self follow-up (+30%)
Probably the biggest winner of T3 -> T4 upgrades. The complete garbage Siegfried gains lots of great stuff. Of particular note is the guaranteed crit skill, which basically just gives you outright +20% damage. This can be very useful in some niches.
Aside from that skill, just overall decent. But decent isn't good enough for T4 costs, so if you're using her, you're going to use her BECAUSE of that 20% more damage. Which is a good enough reason to, really.