There is only a knockdown chance when you hit the head or the legs, and the ranged stun works only if you hit the legs or deal a ton of damage. Instead of aiming for the legs, you should aim for the head in both cases (blunt melee weapon or projectile) because the difficulty to land a leg hit is almost the same as landing a head hit, but head hits tend to be more deadly. Leg hits still sometimes happen by mistake, but they aren't really intended by anyone to put it straight.
Assuming that good head hits one hit kill the enemy in most of the cases, there is barely any room for knockdown and ranged stun at all. I think that both abilities are crucial for the weapon category; archers with low-tier bows have the primary purpose to counter other ranged players or lightly armored players of course, but being absolutely useless against heavily armored infantry is pretty pointless because nothing is THAT dedicated to only one purpose. Consider the poleaxes for instance, which have the primary purpose to slaughter lightly armored enemies and horse legs with their extremely high cut damage. Yet most of them have a secondary mode with blunt or pierce available. Increasing the likelihood of ranged stun would give a bit more flexibility to low-tier bow archery gameplay; I am not talking about bringing it back as it used to be some patches ago, but as Steeve suggested, a STR/IF dependency can make low-tier bow archery at least a bit more effective against heavily armored agibuilds.
Similarly with knockdown, I think there should be a 1/3 of the chance you get now for head hits and leg hits also applying for body hits. Otherwise, knockdown only has a practical effect when the enemy is a tank who survives a head hit with your Warhammer etc. or your character is so incredibly weak that head hits don't even kill lightly armored enemies. If possible, some STR/IF dependency would refine the knockdown system remarkably as well.
But right now, ranged stun and knockdown are so rare that they basically don't even exist. The aforementioned leg hits are the result of bad aim, and head hits should be lethal in the first place.
