What are the drawback of the plate armour knight?

an armored knight expected their horses to carry over 3 hundred pounds into battle...this meant they were not traveling far or fast,which an army must do to win...once they were unhorsed,they were done for...by contrast,the Mongol Horseman and his light armor could travel a hundred miles in a day and still be ready to fight...european armor was no defense against a mongol bow,which could european armor from 300 yards...most arrow wounds were made deadly when they pulled the arrow out...actually the gun was not the death of armor it was Mongol arrows...the mongols destroyed all European armies sent against them...they wore a leather outer coat,with a silk shirt underneath,which was not only light weight,but an arrow would drag the silk with it into the wound...the arrow could then be twisted out without much damage,whereas an armored Knight would be dead...