Could you elaborate more on your criticisms?
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The AI nations seem to be able to ignore manpower constraints and also just get divisions when you declare war on them and the economy has no effect on the game beyond canals and ports. The combat overall is very unintuitive and seems to break reguarly when you fight for more then a few days, with units still having the movement lines but not progressing or damaging eachother. All of these combined make combat incredibly annoying to figure out for me.
The AI nations do not ignore manpower constraints, they just deploy more divisions when you declare war on them, as you can as well. Economy does matter, it's required to resupply divisions. Divisions will wait to go into a battle unless they are fully supplied, otherwise nothing will happen (as per why the units have movement lines but don't damage each other). You can refer to the tutorial for more details.
I do agree with you however that this is not communicated well outside of the tutorial, so I will work on improving that.