I would always use SS for anything that creates a pan sauce.
Nostick just doesn't cut it for that purpose, IMO.
I agree with Mish about that "light" angle specifying the nonstick skillet.
I have had chicken stick sometimes with my SS (like yesterday when I mad coq au vin) but a wiggle or two usually takes care of it.
The bitter can be (1) burned something, like the herbs or onion (2) orange pith (3) the horseradish. I have found that horseradish sometimes tastes bitter right out of the jar even when it it pretty fresh. Did you taste it first?
I also suspect the mustard/horseradish/herb combo may have had a sort of bitter flavor. I don't think it was the chix ...