How good or bad is the penetrating legendary? Is it just affective against enemies wearing armour? Or is it affective against all enemies?

It's not great unless its a radiation weapon

Pretty sure it works on everyone, and it's a decent effect. Not the most deadly but also far from most useless. It's not situational like say 'Nocturnal' or 'Junkies'.

Ehh, the wiki explains it a whole lot better than I ever could.

"The Penetrating mod's effectiveness, due to the exponential curve of armor based damage reduction, does not vary with the target's armor after a certain threshold. When the target's armor rating is less than 15% of the weapon's damage, which is very little, the damage is applied in full (meaning 99%) and the Penetrating effect wasted. If the armor rating is between 15% and 21.4%, a Penetrating weapon will still do full (99%) damage while a standard variant will already experience damage reduction, making the Penetrating weapon increasingly better by comparison (0% to 13.9% more damage). Past this point, which is still very low for semi-automatic weapons, having the Penetrating mod means a static 13.9% increase in effective damage."

What I'm curious to know, is how much effective damage increases after you stack the armor-piercing effect on automatic receivers, or the 30% penetration added by Rifleman.

Cheers all.

It's pretty effective on melee weapons where you can either add a mod for more armor piercing or pair it for bleeding.