The flamberges were never used in combat, they are cerimonial swords used in parades to display wealth and prowess: the flame shape of the blade was very difficult to forge, and only most proficient bladesmiths were able to do it, charging an hefty sum for it. Plus they are way bigger and heavier for most combat use.

Landsknechts used very big two handed swords swinged horizontaly at head heigth or above to cut the enemy pole weapons. As such, they were supposed to die quickly (two days of combat was the average), and they received double pay, double rations of food and better barracks. So the sword was as cheap as possible, because it was likely to be missing at the end of the battle. Plus the flame shape is not very sturdy and its quite impratical, its only really beautiful.

Source, I’m an HEMA athlete in Italy for an association that promotes the study of renaissance combat.