...and I will add that you can have the AI Build Wonders at the Start of the Game even at War. Wonders will Not place Improvements unless the Wonders are Built in Game (Not Pre-placed). Then you can have the Wonders Pre-place other Improvements in All Buildings and or All buildings on the same continent that you can assign a Required Resource so each Improvement will start making Units you want when the Resource is Gained later.
The Great thing about having Improvements placed by Wonders is The AI does not have to take time to build them, the Improvements cannot be Bombarded away, you have more control over the Units that are made at particular times in the game and because the Improvements will all Start Making Units at the Same time, you will have many Units all at once at the intervals you set for them to Auto-produce the Units.
Another "perk" is that Flagged Settings for Wonders and Improvements will all work before the Required Resource is gained to start the Auto-production of Units.
Therefore, if you want Many Units, then Many of the Same Improvement are far better than just a Wonder that can only Auto-produce 1 Unit at a time.
Having said the above, you can also have each Wonder Auto-produce different Units you want at different intervals than you have the Improvements Auto-produce. This also provides more variety and control.
You need to make the Wonders provide What the AI Wants, Gold, Food, Units, Culture, etc... and set the Wonders very low cost so the Build fast to avoid the AI switching to building a Unit when at War.
I believe the programing has the AI build Wonders more from the Start of the Game and if the AI City is not be radically Attacked when the Game Starts, the AI will build the Wonder.
The Main Drawback to having Wonders Place Improvements is you Must set the Cities that Build such Wonders VERY Strong to avoid the City from being Captured before the Improvements Start making Units.
Generally best to also allow the same Improvements to be directly built.
So Bottom Line is you can also use Improvements to Auto-produce Units that a Wonder places in All Buildings and or All Buildings on the Same Continent.