Plate armor is really superb protection aganist melee weapons, and even more aganist scratches, bites and punches. Some proper made, hardened steel, 1,5mm thick plates can deflect 9mm in some cases, or at least lower the impact. Its muskets and other long blackpowder guns like jezzails that ended era of knights. And when we talk about blunt damage… Maybe steel isnt best energy absorber (still better than for example ceramic plates), but it can spread energy of impact over much larger area. So if youre hit with baseball bat, or hulk punch, you will be much better wearing steel plates, even if you absorb the same amount of energy, it will do much less damage when spread.
On the oposite, things like padded armor, gambesons etc. dont spread the energy of impact, but they absorb it quite well. And when you combine these two things… you get very potent combo. Energy distributed over lets say 10x times larger area means 10x more of padding underneath armor can work to absorb damage. But we dont have mechanic in game that would allow certain combinations of armor work better than when separate.