I can't help but think either Suishen was reworked before final printing, or authors beyond book 2 were not given the stats for what the sword would be capable of....we need to come up with a version of Suishen that is still awesome but also not a challenge/adventure ruiner. And maybe. Figure out a better progression order than the vague slaying of servants of the Five Storms.
I used a 10 step progression order with better-defined progress. It's toned down at the beginning but mostly stronger at the end (he becomes a +10 equivalent weapon, but the limitation I put right away that his spells only work on the one who carries him and end if you drop him or hand him over remain). They seemed to make him much more usable and less disruptive.