Horses for courses. Of course European swords are better at piercing armor, that what they were made for. That being said a Katana or Yedo can still pierce just not as well. A European blade is not very effective at slashing. You can surley cut someone that way with it. But it wasn't designed to do so. Where the curve along with the shape of the edge of a katana will cut through a body easily. With that said a knight with full plate armor was a sitting duck while off his horse. The battle of hastings proved that one. The French knights in full armor were taken down by archers using their daggers. Which eventhough I like using ebony and plate armor in skyrim, it is nowhere near realistic to be running around all day in the stuff.
Against an unarmored opponent, the longsword was more than sufficient. It can hack off limbs as easy as any other sword, even the overrated katanas (which were actually pretty bad). Against metal armor, slashes were
completely ineffective. And no, Katanas can't cut armor either.
Nearly every fight between armored soldiers ended with one throwing the other to the ground where you could either bring your whole weight on the tip of the sword to penetrate the armor or you went for the visor.
And you absolutely couldn't do that with a katana. For one, the curve and the broad tip made it rather hard and for the other, the katanas would break.
The katana was designed to cut through unarmored peasants and unarmored duelists. In a real battle, any sane samurai would prefer a big club to a katana.
The Longsword on the other hand was still usefull against armored opponents if used with the halfswording technique (grab at hilt and halfway up the blade, then stab). The sword guard was also used as a pretty effective bludgeoning weapon.