Thing is, I don't get the impression that Radiant Blade wasn't meant to be a heavy hitter. Let's compare it to Rean's Flame Dragon.
Radiant Blade
Magic Attack (Set) - Area (M) - Mute (90%)
Unbalance +20%, Class S, 40 CP
Flame Dragon
Attack (Set) - Area (M) - Burn (50%) - Faint (50%)
Unbalance +30%, Class S+, 45 CP
Both hit the same area. Flame Dragon is slightly more damaging and unbalances more consistently. Radiant Blade is slightly cheaper and more reliable at inflicting ailments (Flame Dragon has a base 75% chance of inflicting something). They're very close to each other in terms of power, but only if Laura's ATS is comparable to Rean's STR. If Radiant Blade is supposed to consistently deal less damage than other S-class Crafts then it's overpriced in terms of CP cost. This is what makes me think that Falcom wanted Laura to fill a different combat role in CS II, but didn't get the balance quite right in time.
You're comparing it to the wrong craft. Laura's goto AoE craft is Radiant Spin:
Power: S+
Range: Area (Large) - Set
Effects: Draws foes towards Laura, Faint (50%)
(50CP)
The literally did not change Laura's role. She has the exact same crafts as before, only better, with buffs to her primary offensive crafts and S-craft making her even more powerful as an offensive tank. The intent with Radiant Blade is to target magic users and cripple them with Mute. Obviously this doesn't work out in practice, because most bosses resist Mute and you're better off outright killing trash mobs with Radiant Spin. The fact that it works off her ATS is bizarre as well, but I suspect they wanted to give her an option to target enemies with low ADF, in the event that you run into high defense enemies. Doesn't matter because Laura's Attack is so high, she ends up doing way more damage against high defense enemies anyway, lol.