I'd like to know which games are the 30% that IGG is ignorable. I found it is very hard for me to understand. Somehow it is a lot weaker than other cursers vs. trashers? (I don't mean in the same board; instead, there seems to be lots of boards where ok, I am going to get a witch even with this trasher available, then if there is IGG instead of the witch, it no longer worth getting.) Am I right at this?
Games with Traders. This is the big one. You spend $5 for what is essentially a self-duplicating Copper, and you're possibly handing me a Silver in the process. Not only that, but when it happens, the piles become uneven. No curse is given or trashed.
Games with Watchtower Not only can I trash the curses as they come in, but I can also buy curses, trashing them immediately to make the piles uneven (obviously works best with some source of +buy).
Games with Ambassador. I give the curses to you, and make the piles uneven by deciding how many to return to the supply.
Games with Embargo. You hurt yourself as much as me by buying IGG and make the piles uneven.
Games with heavy trashing and a dominant engine. Something like Chapel, Highway, Grand Market.
Games with hand size reduction attacks. I haven't tested it, but I assume Militia or Ghost Ship would do well against IGG. They mitigate the effects of curses while making it harder for you to get $5 with Copper/IGG.
Games with Jack of all Trades, possibly. Playing JoaT to trash curses while gaining silvers is nice. You'd use the Silver to start buying out the Duchy pile before the other guy was done with IGG, beating him to the punch.