The one unit per stack thing really shows it's flaws when you have lots of large civs fighting across a landmass though. The land can become 'clogged' with units to the point where it can be hard to move around properly.

Couple that with Civ V having possibly the worst AI of any Civ game to date and it's a recipe for frustration in many cases. The AI civs would never use naval units properly and it made a large portion of the game easily expoitable.

Civ 4 was the other extreme, where with many large civs you would end up with the 'stacks of doom' on a single tile containing dozens of units. Obviously this wasn't ideal either.

Some sort of unit stacking limit would have worked I think. Some mods exist that allow you to stack between 2-5 units on a single tile in Civ 5 but I'm not sure how well they work or how the paltry AI copes with the change.

Still, I always felt like stacking by combined arms would have been better, like a stack of 5 units limited to 2 inantry units, a cavalry or vehicle unit, a ranged unit and a siege weapon.

I always found it amusing that in 5, archers ('missile') had a longer range than any gunpowder longarm ('infantry'), from muskets to modern rifles. I understand the gameplay concept, but it's just stupid when some ye olde archers can hit your riflemen and they can't hit back. I actually think that the transposition of unit tactics onto the world map doesn't make good sense at all; it's an ersatz experience.