As far as I know there hasn’t been any ai changes, but there’s been a number comps added, namely any of the ones that use SC1 units. But the big one was that on hard and brutal Amon’s units started to get ability upgrades (like Stim and Combat Shields for the Marines)
per this update: Co-op Mission Update - June 13th, 2019
As well, many of the buffs over time for CO units got added on to Amon’s versions, things like the Yamato charge time.