I've only just started playing Rome, but I've had a 32-unit barbarian stack come through in only year 2 which, when you've emptied Rome like I did to take on Carthage, is a real killjoy.
Also, if you start Selucid, Egypt immediately throws a 50-60 stack army at you in the first couple months, that got my attention right quick.
When you defend/attack Egypt, you see how thinly stretched the Sel empire is, as your manpower goes from 100k to zero crossing into Alexandria, and that's playing with a 25% easy bonus, just because I like big army stacks without headaches. It's even worse if you try moving armies by naval-fleet, they get hit with 5% the second they board the ships, which I think is a bit rough.
Lesson learned: Big stacks are as dangerous to their owners as they are to their enemies. I generally keep mine in the 10-15 range, generals permitting.