I don't think I'm good enough to talk about ICS in general but I think I can talk about ICS in Civ5. ICS does not feel like an exploit. You don't have to go out of your way like Emsworth agreements and the resource disconnection exploit. You just get lots of happiness from intended sources and spam out settlers from a specialist city. This would be pretty obvious looking at the game rules from the outside for G&K. There are many new sources of happy like more luxs on the map. The only deflation that happiness suffers from is piety being nuked. However, people where always going rationalism anyway so that really does not count.
The big problem with trying to stop ICS in Civ5 is that to make it balanced the happiness cap needs to be made much tighter. This would not work out though. You would have to have like -8 unhappiness per city. Almost all players would not be able to deal with that. They would get stuck at 2-3 low pop cities and fail. Less than 4% of players have beaten EMP. Don't tell me that everyone quits before they win because more players have unlocked all social policies and beaten the game with many civilizations. On a side note this is why MP was not cared about at all. Its kind of pointless to play it when you cannot beat EMP.
The big problem with trying to stop ICS in Civ5 is that to make it balanced the happiness cap needs to be made much tighter. This would not work out though. You would have to have like -8 unhappiness per city. Almost all players would not be able to deal with that. They would get stuck at 2-3 low pop cities and fail. Less than 4% of players have beaten EMP. Don't tell me that everyone quits before they win because more players have unlocked all social policies and beaten the game with many civilizations. On a side note this is why MP was not cared about at all. Its kind of pointless to play it when you cannot beat EMP.