Most of the AI vs. enemy player differences come down to the AI "cheating" but being unable to perform complex tactics the way a human can. (Any strategies related to humans here are assuming the human player is at least halfway competent. Some players can be worse than the AI at times.)
Some examples:
>(Cheating) The AI can train spies/saboteurs/infiltrators (or presumably the Foehn equivalent) as soon as it builds a barracks. The human player has to wait until they have a tech centre (or equivalent). This often means that the first AI unit you see is an enemy spy trying to sneak into your base - so make sure you train those dogs in the early game!
>(Cheating) The AI has unlimited money. A human player actually has need of those miners. This is how the AI can tech up faster, train more units, and can afford to send all those spies at you in the early game. For this purpose you may as well set your starting money to max if only against AIs, just to level the playing field a little.
>(Cheating) The AI knows exactly where you are at all times. Granted, the better human players will have memorised the starting locations and will thus have a good idea of where you are, but they won't know exactly. And moving your MCV at the start rather than setting straight up can fool a human player (although it's risky; they'll have to spend time looking for you, but you'll be behind on base and army building), but won't fool the AI.
>(Strategy) If up against multiple AIs, there's a distinct "gang up on the human" mentality in play. Any AIs not directly allied with the player will treat the player as a priority target, even if there are other, weaker AI teams to go pick on. An AI will typically only start prioritising another AI if it is positioned between you and them. Best seen on the "No Base" skirmish type; every single enemy AI will immediately head straight for you and dogpile you, and only when you're defeated will they turn their guns on each other. For this reason the No Base skirmish is effectively unwinnable if you're up against more than one enemy, even if you've given yourself AI allies. A human enemy will meanwhile go for targets of opportunity, and probably start with the nearest player, AI or not.
>(Strategy) The AI has little concept of effective base design and will scatter its structures all over. A human player will be a bit more intelligent and focus their defensive turrets along likely avenues of attack, refineries near the ore fields, and (depending on the player) will actually put up walls to stall you in front of their turrets/other defences (an AI will only wall off the Construction Yard and maybe (the Latin Confederation seems especially lazy here) its Tech Centre).
> (Strategy) In terms of general strategy, the AI is predictable. It sends groups of units, mostly of just one type per attack, via the most direct route to your base... although it does sometimes look for the least guarded route to actually get into your base (most often seen with enemy spies knowing where your dogs/terror drones/robot tanks are and avoiding them like the plague on the way in). If you can see what's coming, you can easily move the appropriate counters into place and decimate them as they arrive. To give the AI credit, it does sometimes look to see what you've most focused on/are lacking in choosing its own unit selection (e.g. prioritising gatling tanks if you've got a lot of infantry, spamming aircraft if you're lacking anti-air). A human player will try sneak attacks, pincer attacks, uses mixed unit types per attack group, directly manages their units during an attack to ensure each one is fighting the most appropriate target (whereas the AI either prioritises "nearest unit first" or "unit with least remaining health first"), changes attack intervals to allow for different attack sizes, actively goes after targets of opportunity, baiting out the enemy defences (although that shouldn't really work against a human player), etc. In short, a human player is unpredictable and won't keep trying the same tactic over and over if it isn't working - and every player has different preferred strategies.
>(Strategy) Related to the above, the AI is simple minded when it comes to superweapon usage, and always fires at the greatest value of units/structures. While a human enemy may do that as well, they might also have other goals in mind; e.g. wiping out a good section of your power plants in order to disable your turrets as they launch a conventional attack.
>(Strategy) Also, the AI never guards their ore miners (which don't really matter because they have infinite money). Human players will, and may outright put up permanent garrisons or outposts near outlying ore fields to secure them against you.
>>In fact, the AI doesn't really bother with proper guard forces at all. If you roll into their base, chances are you'll mostly be up against turrets and whatever units the AI has built but hasn't yet sent against you. Human players will at least have something substantial on defence, but the AI doesn't tend to leave more than a few anti-spy units and maybe a few infantry.
>(Strategy) Strangely, if you don't build a shipyard and start deploying naval units, the AI rarely sends its own naval units at you - even surface bombardment vessels like the dreadnought and aircraft carrier just seem to stay at home when they could be bombarding your base to bits with little opposition. Of course, try ignoring the naval side of things with a human player, and they'll just see a great big hole in your defences to exploit.
In short, the AI is relentless (if occasionally flawed), but hardly as interesting to battle as an actual human player. A human player will get creative and keep you on your toes.
