There is definitely a problem with difficulty scaling, just put up 1 hardest AI against 2 easy AI and see it struggle. In every other RTS game the hardest AI would easily annihilate the other two.
Also AI only matches are pretty boring to watch in AOE4, as they do not commit to an attack and always re-evaluate their engagements…
Hardest french AI (now grey) losing to easy english AI ram push
From my experience with these AIs, I think Easy is actually amongst the best of them. Sure, he’s low econ, barely ages up and spams spearmen at you, but at least he actually attacks. And the AI is particularly bad at fending off early pressure (as are newer players that are likely to pick Easy AI) so he’s really strong against other AIs.
The AI is broken 
I hope the next patch will fix it
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A.i needs big improvements.