but a stronger one is discard/masq. Something like Ghost ship/Masq can put a Fg player into the uncomfortable bind of having to either hand over a Fg (they keep 3 Fg in hand) or never buy a province again (they keep 2Fg and something else). Ghost ship is truly vicious here as they will eventually have to hand over VP or Fg.
A rabble/masq engine can also make it extremely hard for a Fg player to keep both their VP and their Fg.
It seems to me that the fact that Discard-Masquerade and Rabble-Masquerade are very strong doesn't have much to do with Fool's Gold. Almost any deck will have an extremely hard time against someone who can play Discard-Masquerade consistently. (Except maybe decks based on "draw-up-to-cards like Library or Watchtower.)
That goes especially for "Something like Ghost ship/Masq can put a Fg player into the uncomfortable bind" - Ghost Ship-Masquerade simply is one of the meanest combos out there.
There are two big ways to hold up against discard/masq:
1. Pile out quickly. With no other trashing (and no Tr/Kc/etc.) it takes a while before masq can thin down to a reliable engine. Yeah, it isn't exactly slow, but fast combos can win before it can strike too often.
2. Have a deck with just a few key cards that can nab provinces with just two of them so you can keep a copper around. For instance, golem decks are quite strong against masq/discard as you can keep a copper (or whatever) and your two important actions (golem/whatever) and play through most of your actions. Rebuild can similarly work to get a province off a one card play. Other digging cards also work (e.g. Hp). Limited draw cards like jack, Wt, and Lib all can get you back to a province buying hand and let you keep a copper in hand. Big draw like menage, hunting grounds, tactician, etc. can also let you keep around a copper and still kick off your own engine or hit enough cash to buy a province. Alt-VP can also be quite resistant - horse traders/duke has copper to keep and can hit 5 coin easy. Other shots like Iw/Gardens or Count/Silk Roads can also do pretty good at gaining more VP cards with a single action play - leaving 2 other slots in the 3 card hand for copper to be held.
Fg is fast, but unlike some other fast combos (HoP, Bridge, etc.) it slows down massively if has to aim for 8 provinces. It only depletes one pile itself, and tends not to easily pile out something else. Fg is normally the money strategy most hurt by green - not only does green increase your deck size, but it lowers the density (and hence expected value) or the Fg. The longer the game goes, the more time masq/discard has to force your economy (Fg & gold) or your VP into your opponent's deck.
Likewise, Fg is a card that is nearly useless in a 2 card hand. Very few of its enablers will let you get out of a 2 card hand with a province.
Is Fg particularly vulnerable? Debatable, though I'd lean a bit towards yes for the problems with density, game length, and lack of use in 2 card hands. However, discard/masq isn't a panacea and is strong enough to merit (sometimes) going for an engine against Fg.