What is the other "flaw" on Callandor?

It gives you a headache.

 

It's a sword...but it's not sword...but you just said it was a sword...and it is! except that it isn't...but-, oh forget it!...it is shiny though :)

IMO, it makes you vulnerable somehow to attack.

I believe Min seems to think it will leave Rand open to some sort of outside control. Possibly like how channelers are open to the 13x13 trick? Recall the line of prophecy about how all that is the wielder of Callandor can be seized, and Min's viewing of an onyx hand holding the sword.