Yes,plate armor wasn't nearly as common as a lot of people think back in the day.
it was,however,a lot more effective than people think.One man in full plate and chain,backed by his men,could cut a path straight through the opposing army.
With the introduction of surface hardened steel,swords and even the heavier blows of axes would deflect off of the plate.
A charging horseman with a lance may very well pierce through the plate,to be stopped by the chain and leather underneath.
A hammer or axe,with a small edge designed to pierce through the armor,or transmit the force through it,was effective,but modern tests show that multiple blows in the same area with considerable force were required to significantly hurt someone underneath.By that time,the man inside the armor has surely killed you.
Arrows from bows show little impression upon the steel,and crossbow's,while able to pierce most plate of the day,were often slowed enough by piercing through the plate that they would do little to the man underneath it.
Not all is created equally.some plate is thin and to be worn with chain mail and leather,some is thick and tempered,meant to be worn alone,so on so forth.
A breastplate and helm were often the only piece worn,and even then expensive,meant for military commanders.A full suit was often commissioned only by the very wealthy,worn by their sons and some very well-to-do knights.
Realize also you are not targeting a dummy with armor.This is a man,who moves and dodges around(plate armor is not as heavy as you think)and if you miss your first strike,you very well may not get another one.
To sum up what I'm trying to say,a combat experienced man with a full suit of plate armor,mostly against conscripted militia and archers,is an absolute juggernaut.They were not brought down easily,more often the tactics of throwing people to the slaughter in the hopes one would strike a vital area.
That said,it was a rare sight to see,and by the time many blacksmiths were to the stage were they could produce the armor for more than the wealthy,the implementation of gunpowder brought it to a halt.
Just something for you to think about the next time you send your fully iron plate wearing dwarfs out to slaughter goblin siege number 243.