Fg is a card that is nearly useless in a 2 card hand.
2 FG = Duchy. Few other cards can even get that much in a 2-card hand. Gold, and... that's all I can think of off-hand, unless you're drawing some more cards (esp. with Library and such, but of course Library can be mixed into a FG deck). Surely this makes FG one of the best cards if you're playing 2-card hands!
Edit: Of course this doesn't help you a ton if your opponent is reducing you to 2-cards every turn, but if there's discard+masq and a reliable engine on the board, why on earth are you not playing it?
Fg is only close to a reliable duchy when 40% of your deck is Fg. For a straight, uncontested Fg rush that is true only up until you have 5 cards besides Fg and starting cards. Even when the percentage is higher than 40%, because you can't use the high payout of 3 or more Fg in hand, you won't even average a duchy every turn.
So yes you can buy a useful 5 when you hit two Fg and you can buy a duchy, however when you hit just 1 Fg (which will happen quite quickly when you buy duchies and are forced to give away Fg) you can buy an estate.
Smithy gives you a 4 card (no action) hand and that beats Fg most of the time when your cash average is 1.25 coin/card (i.e. the minimum I would expect every BM-smithy deck to be hitting by the time discard/masq becomes reliable). University, menage, golem, etc. all do much better than Fg in 3 card hands. In some circumstances, things like Bishop and Monument even come out ahead (e.g. 3 player games where both of the other two players are playing discard attacks). I'd say just about anything that will, long term, bring in 2 VP per turn on average should beat Fg here.
There are several reasons not to play discard/masq yourself. The easiest one is if you are playing 3er and the opponents are not symmetrical. E.g. if the guy passing to you is going for gardens, you are going to wait forever to see any useful VP; likewise if the player to whom you pass is going Xroads & heavy VP, you really don't want to make him discard & pass to the other guy. Likewise, if you look to get the bum end of curse passing when one player (and not the other) has gone jester. It is quite possible & likely in 3er or higher that the player helped most by discard & masq won't be you.
Another reason to skip out on masq & discard is possession. This is a tricky one as discard & masq can often get to possession first, but the risk of 20 VP swings can make having masq in your deck too dangerous if he can just beeline possession before you can get there (e.g. golem/remodel).
There are a number of things where discard attacks are just too helpful - library, tunnel, watchtower, beggar/feodum, and even some countinghouse setups - and your return is just too low.
And of course there is the thing with faster attacks - like curse/ruins/copper givers that make it hard to build up to strong engines and get payout before the game end.
Yeah masq/discard is wickedly strong, but there are number of niche strats that can beat it and beat it handily (e.g. Possession, P-stone, Tunnel, Minion).