try comparing the fan of knives to itself using different weapons. like the difference between using captain's greatsword and two kunais.
the lack of information on the skill is kind of frustrating.
Given that Fan of Blades seems to work exactly like Cleave using two Daggers, that would be my point of comparison.
Is using Cleave with a Captain's Greatsword better or worse than using Cleave with two Daggers? Technically it's just different, but I would personally say that using two Daggers is superior. More damage base damage (53 instead of 50), more Armor Penetration (10 instead of 0), more Evasion (+5 instead of -10), much more reliable damage output because there's twice as many attacks (a hit or miss does not affect the total damage as much), it's Ranged so it doesn't trigger retaliation and doesn't have a hit penalty against flying things... the only drawbacks is that the Daggers have only 10 Crit instead of the Captain's Greatsword's 20, but that's a tiny sacrifice compared to what you gain.
Not gonna lie though, I've always thought the Captain's Greatsword was bad, like the Scythe. Galon's Griefmaker is just way better.