Civ III AI also lands troops amphibiously at different points, causing you to split your defenses. (CiV's AI could learn a trick or two about naval warfare from the Civ III AI, that's for sure!)
The AI in Civ III always knows exactly where you have your units. If there is some point in your empire weakly defended, then that's where it'll land its troops, guaranteed. But this gets predictable. Yes, you need to hold some units back to deal with these landings, but a couple of fast units will do, the AI doesn't attack with big forces amphibiously.
The trickiest points to defend against the AI in Civ III are islands. The AI loves to target those, and it's more difficult to get your units there. But the amphibious AI in Civ III is not a real force. It has merely been designed to keep the human player busy, I remember having read the AI designer himself saying something of that order.
Apart from the amphibious aspect, the human can build its tactics upon the knowledge that the AI will always go for the weakest point. The whole 'bikini babe tactic' was based upon this. The AI would try to move
around a wall of your own strong units, just to get to some undefended worker behind it.
The AI in Civ 5, yes, it's certainly hit and miss, you're not wrong. I often see hilarious things, like an AI having a City State down to the last hitpoint, and each turn again firing on it with a large battery of ranged units. Oh AI, if you would only take
one little puny melee unit over as well, that town would be yours, but with
only archers, no, not in a 1000 years you will take it!
I've seen both England and Arabia being guilty of this, probably not coincidentally both civs with a ranged unique unit. There's probably something in the build flavour of these civs that makes them produce lots of ranged units.
The AI will also easily split its troops, like half a force will charge to one of your towns and the other half will go to a City State you're allied with and neither of those units are getting anywhere, because they're not attacking in a concentrated manner.
I don't think the rule changes that are being proposed now are primarily meant to help the AI. With longer health bars battles will last longer, but it'll become easier to save your units by retreating them in time. Will the AI learn to retreat its unit at the right moment? Big questionmark here. Maybe they will, it doesn't seem unprogrammable, but I've gotta see it first.
Embarked units being able to defend themselves will be good for the AI, but the current system is silly anyway. Embarked units are becoming defenseless and can't even be stacked with military vessels. The AI is going completely wrong with this, but also as a human player I'm not pleased. If I'm getting a military unit from an island CS I can't get it safely to my lands. Building 6 frigates and having them form an escort would be the only safe way, but that would be bonkers. No, that change is very welcome, for both the AI and the human player.