In an ideal world Firaxis would have a few employees dedicated to programming the AI, considering that it's the mainstay of the single player experience.
However, we know the AI programming tends to be a side role of a single programmer.
Stacks of Doom in Civ4 were easy for the AI to cope with, making Civ4 a game with good AI.
From what I'm reading, I can't tell if one unit per tile is completely gone - these new army systems sound like they could be anything from a bandaid on 1UPT to the best system we've ever seen.
I'm still disappointed that we don't just have the system from SMAC but with hexes, where you can have stacks of doom but when a unit dies it damages all other units in the stack.