My friend and I have played about a dozen games of Roma, and it seems pretty consistent that the player with the most forum cards wins. They're just the fastest way to collect victory points. Has anybody successfully fought back against a player who has more forum cards?
Mercator is usually a sure fire way to beat a forum machine, but there are defenses against that too. (Siccarius and Gladiator both do very well.) If you Haruspex the Mercator and you have a Consiliarius and/or Scaenicus in play you can usually win handily against a forum machine. This will usually force your opponent to play offensive with Legionarius and the like. (The money you use to buy VP can quickly turn against you if your opponent gets decent draws - but they have to realize it and stop spending their action dice on VPs and draw cards or else they will likely lose.)
The Mercatus though isn't so good since you only get points for the # of forums your opponent has, while your opponent is typically generating more VP than they have forums on average. It could be good in some situations but I personally have never seen it shine.
(Forgive my spelling of card names, I don't have them at hand to verify.)