It's actually historically inaccurate to say that bludgeoning attacks were particularly effective against heavy armour as the armour spread the force of the blow. Unless you struck the head, of course, but that's the same for all armour. Similarly, piercing attacks tended to slide off unless you got a good hit. The downside of the armour types is modelled by the maximum Dex bonuses allowed.
That is incorrect. Warhammers were historically a primary weapon over swords against armored opponents. While you are correct that a heavy armor spreads the force of blows, that also applies to cutting and piercing damage that uses less force because the balance of the weapon is near the handle. Solid plate armor turns a direct slashing or piercing attack that is not precise enough to target lower armored area into bludgeoning damage by spreading the impact of the edge. Since the weapons use less force when the force is spread is converted it ammounts to less bludgeoning damage and so less damage to the target. Warhammers were meant to maximize bludgeoning force by balancing the weapon far to the head ignoring the transfer into bludgeoning damage and killing the opponent by maximizing force breaking bones and causing internal bleeding. Polearms were very popular often having and axe for unarmored and bludgeoning side for armored opponents because hitting an armored opponent with the axe side might cause it to glance because it needs a more precise hit with proper edge alignment to be effective where dropping a heavy flat surface from twelve feet up on an Knights head is likely to break his neck because the helmet is suspended entirely on the neck. Most armor does not have a neck brace you simply wear the helmet so all the force from a drop from above is transferred down into your neck. A rounded helmet can cause arrows blade edges to glance and transfer a large portion of the force to the shoulder armor reducing the force of both impacts or deflect it to the ground meaning the knight is not taking the majority of it impact in any way.
I am in no way saying piecing weapons were not used but as armor got better shield got smaller because Knights could take a direct arrow hit and it simply didn't have the mass to generate enough force so a small shield to cover eye slots were all that was needed. If your attacking with the spear rapier you aim for lower armored arm pits and other joints, cutting edges need to aimed at the neck to try an cut under the helmet and above the chest piece. That is also why many shoulder guard are designed to shield the neck and fanned armor to keep piecing and edged weapons away from the joints.
The truth is your better of with a Warhammer, mace, or bludgeoning polearm against a heavily armored opponent.
Slashing damage is better for cutting aches which means focusing force to an edge for penetration and maximizing areas of impact with that penetration. It
requires less accuracy then a penetrating weapon or bludgeoning weapons the are most effective in transferring force a the initial point of impact and less strength and force for fight endurance than a heavy bludgeoning weapon making it ideal for sustained combat by skilled and unskilled swordsmen vs unarmored opponents where any blade alignment is likely to sink into the target and guide the blade instead of deflect it.
Piecing weapons are precision weapons using focused force to allow the user to dramatically reduce the total strength and power needed but requiring a
much more skill. This means its efficient and better when you have your opponent by surprise for a still target making hitting a precise vital organ like a knife stabbed into the kidney or heart from behind, a throat stabbed in the side from behind, or an arrow/bolt into a target standing still giving a speech. When make these attack you either need a high degree of skill or a for your opponent to provide good opening. A flesh wound with a piercing attack would typically be less damaging than a bludgeoning or slashing damage. While a successful targeted hit on a vital organ is more likely to impact the organ and kill the opponent in a single hit.
If I were going to make weapon rules I would do something like this:
Bludgeoning is standard damage (punches, kicks, hammers) and does not change.
Slashing would half a +2 to hit, -2 to damage
(minimum 1) versus medium and
-3 to damage (minimum 1) versus heavy armor.
Piercing would be -5 to hit, -2 to damage (minimum 1) but
if you hit with natural 16-19 you attack does critical damage and a natural 20 would do a maximized critical
weapon damage without the -2 to damage. For example a natural 20 on a 1d4 dagger would do 8 damage every time +str/dex + additional standard critical damage dice. If you have a 1d6 backstab your still it would be a 2d6 for your critical but your not maximizing that since its from a skill, spell, ability, other than the weapon.
However, this would require other balance adjustments because archers for example would critical about a 1/4 of the time and the archer classes were not design for this. However they are going to miss 20% more too so that might already level. Hard to say without play testing.
