aggressive expansion however has one glaring flaw, weak defense and poor response/reaction time due to distances units have to travel.
You forgot the biggest problem with city spam: low growth.
People who keep on saying that city spam solves everything are usually relatively recent players. You need to grab a piece of land, sure. You need to have 3-4 cities by turn 50, or you are not building an Empire, you are hiding your head in the sand, hoping no one will notice you. (Oh, yes, they will)
But to me, it's the same as complaining that you need to research techs, or that you need to adventure with your heroes, or that you need to gather resources. They are ALL part of the game, and you need to balance them out.
Just try to focus exclusively on spamming cities, to the point you have your capital, and a bunch of level 1 cities that grow by 1 population every 5 turns, and see how well you do.
Initial land grab is something you HAVE to do, just as you HAVE to ramp up your production in your first city, just as you HAVE to bring taxes to none, just as you HAVE to grab resources, or LOSE. (At reasonable difficulties. At low difficulties, you can just conquer stuff with your two heroes and a couple of rushed spearmen.)