Honest question... did it get better recently? I tried it years ago... and the AI was non existent...as in, I built up an army and went to explore, only to find my enemy had like 4 peasants.
0AD is definitely not the same game now as it was years ago, it is massively improved. These were both pretty big releases:
https://play0ad.com/new-release-0-a-d-alpha-21-ulysses/
https://play0ad.com/new-release-0-a-d-alpha-22-venustas/
I find the AI to be challenging to beat on hard, but not impossible. I'll play as the Ptolemies, because I know that they were the pinnacle of the Greek Phalanx lead by none other than Alexander the Great. So I was trying to take a large group of spearman up to take over an enemy town center filled with archers but wasn't having a ton of luck. Every time my infantry would retreat, the 20 archers would come out and chase them down. When I'd turn back, the archers would head back to the town center and shred through them. My winning strategy involved staging 10 companion cavalry to the east and have the infantry attack the town center and then retreat south, drawing the archers out to try to take out the infantry. The companion cavalry rode north of the archers cutting off their retreat to the town center and let the infantry cut them down.
The newer AI will also scout out your location and attack weak spots. They'll purposefully avoid attacking through walls and lots of towers if you mistakenly leave an unprotected rear or flank that they can use to march right up and destroy your town. It is certainly more difficult now and the new graphics make it a lot of fun to play.