Idle Master is not helping with card drops. It just fools Steam that you have a game running. After the game has been run a while a card will drop and it all depends each individual game's drop rates.Idle Master isn't dropping cards for any game though. So I will try it out.
There is no difference running idle master and running the game itself. You just don't need to download the entire game just to idle. You could even use SAM to idle a game.
If you don't any card drops then it means that the game you are idling has insane long drop rate