I was rather interested in using the Custom Difficulty proficiency modifier because i thought it affects all forms of attack and defense equally, as an elegant way of just tweaking down some of the rough random rolls (since both karma & no karma dice feel off).
But upon reading a bit more it seems that proficiency bonuses (from character levels) do not affect AC (armor class) at all.
Meaning any attack/spell that uses the Attack Roll mechanic (& goes against AC) would be notably buffed compared to "Saving Throw" attacks/spells.
Because Saving Throw attacks/spells & defenses are both equally affected by proficiency modifiers, somewhat nullifying these bonuses against one another.
This makes it far more sensible to spend your action points and spell slots on stuff that uses (or benefits) Attack Rolls instead of anything that interacts with Saving Throws, unless its some kind of a gimmick encounter.
Feels rather lame that an option whose purpose is to reduce the impact of random rolls (as the description says) would also dumpster spell/attack balance in such a colossal way.
