Am I reading this right?
Masterful Builder says: once per scene, when making a check to use an item you personally crafted, you may add a number of kept <ring dice> set to * results equal to your school rank.
Kept dice are decided after rolled dice. So, if wielding a weapon they crafted themselves, they can wait until they roll enough successes to hit, then make it an automatic crit.
Might they be the best iajutsu duelists then? Especially when combined with the right distinctions, like Quick Reflexes .