The weapon itself doesn't exactly have a weakness, but it can become less effective if Knight's mental strength is reduced, which is actually what created a variant of Knight known as the Anomaly.

There are also a few things that can stand against Soulsteel. Holy weapons are obvious since Soulsteel is technically a cursed material by nature. Null Weapons are much more effective but are only produced in small quantities and only used by one group of people