Governor is nuts with discard, too. I think Governor and Governess both don't cut the mustard for what you expect out of a set card, but meet criteria for a promo card.
I doubt that governess would be even alright as a promo, but I also disagree that set cards have a higher "quality" then promo cards, if that's what you're implying. Transmute, Scout, Hunting Party, Rebuild, Harvest, Pearl Driver, Scrying Pool and many others are all set cards. what's the worst promo we have? dunno, probably Stash, but stash is completely fine.
i don't know exactly what criteria determine whether a card is a promo, but looking at black market and governor I'm guessing it's a question of complexity. WV and embassy could easily be set cards though.
No, I'm not saying set cards are always better quality than promo cards. From Donald's secret history posts, I get the vibe that he just has different goals for promos and for set cards, and that certain drawbacks are more concerning with set cards versus promos. Envoy and Walled Village have a lower criteria for "similarity to already released set cards", but Walled Village has a higher criteria for thematicness (dunno about Envoy). Black Market had the bar lowered for how much complexity is acceptable.
What Governor had a lower bar for is the acceptable level of dominance, and when you compare it side by side with Lab it's pretty clear that the card is deliberately situated on the strong side. Governor's interaction with Militia and friends is part of that acceptable dominance, because it's a promo, and if you acquired a Governor you probably wanted to play some Puerto Rico during your Dominion game.
To be clear, Governess is probably not high quality enough to be a promo card because it's not interesting enough. Its dominance in discard games isn't what flunks it out, though.