Masquerade:
If i'm not forgetting anything, there are only around 3 4 5 handsize reducers that leave you with 3 cards (Militia, Goons, Mercanary, Ghost Ship and Margrave) and 2 that leave you with 4 (Urchin, Minion), and of course there's Bureacrat, Torturer and Cutpurse (i ignore Followers). All of these are terminal, so you need a KC/TR or Village for discard/Masquerade to work. Most of the games you have Masquerade, at least one of those parts will be lacking, and even more often it will be hard to set-up reliably. Without handsize reducers, Masquerade is nice, but... not that harmful.
Also consider that there are games with more than 2 players (trust me, they exist). In such games, if there are discard attacks, a player relying only on them will usually profit more from your Masquerade than you do, as he doesn't need to set up an engine for it. Now imagine him being the player to your left. With junkers on the board, Masquerade is harmed, too. If player 1 plays Masq, player 2 plays cursers, and player 3 keeps passing his curses to the Masq player, Masqerade harms the curser player and helps his other opponent, lowering Masquerades value at the same time.
Mountebank:
Mountebank is an attack, so everything that counters that directly weakens Mountebank considerably. This includes Lighthouse, Trader, Watchtower, and of course Moat. Also, there are followup-counters like Rats, Jack, Outpost... I remember i once even won against Mountebank with a Counting House strategy (though that might have been because my opponent played poorly).
Goons:
Goons is an attack, too, so Moat, Lighthouse and Watchtower work here, just as do Beggar (which also helps emptying the Copper pile) and Horse Trader. Also there are Library and Jack, and unless there are cost reducers (chances of which are slim, there are 4 if you count princess), Embargo can block cheap Copper-purchases (though this becomes weaker with Cursers on board).
Rebuild:
So all of these power cards have pretty clear situations where they are weaker. I feel something for this is lacking for Rebuild. As it doesn't get its power from harming the opponents, "protecting against it" is impossible. The only thing that comes to my mind are Shelters, buying Duchies out and playing Saboteur a lot, but besides Shelters none of these seem very attractive to me.