They actually have a better (potentially) soak against magic than against other sources of injury. The bad news is that magic damage is pretty reliable, it almost always hits without having to roll for targetting and is usually pretty high. But then again magic damage is also rolled as a stress roll, so that means it could botch (meaning 0 damage).
It depends on the PeCo effect. If the effect is "inflict a heavy wound", and it is not resisted by some form of magical resistance (like Parma Magica e.g.), the target gets a heavy wound, whatever his Stamina score is.
Yep. Soak only ever applies when quantifiable damage is done. Is there no Stamina roll allowed to resist a degree of damage? Sometimes a tough character might be allowed a roll against an ease factor of 12-15 for a lower wound level. Just a suggestion.
It seems to me that a lot of the 'natural resistance' rolls from 4th ed are eliminated here in 5th. IIRC there were a lot of Int+Stress rolls vs. 6 ot 9 to avoid various Mentem effects. But I don't recall anything ever affecting a PeCo that deals a fixed-size wound.
Yeah PeCo is all about : penetrates or not. If yes, you are wounded.
And for that reason, they are more context-dependent (humans against CrIg which works on everything without fire resistance) and higher level (base 30 to kill, and in CrIg, base 30 means +35 damage... which is basically the killing level require +15 damage.