The only legitimately challenging ones to use that I could see are:
- Sofa - the AI doesn’t know its mounted units get a combat bonus near its archer units
- Suffet - the GAdmiral/GGeneral switching is tough to deal with, even for humans
- Armada - the switch to a faster/weaker unit when damaged Vs slower/stronger at full HP is something the AI doesn’t know about. Not sure if it matters
- Tarkhan - the AI doesn’t know that if it escorts settlers etc with their UU they will move faster... It does that anyways, but it doesn't know this unit does it Particularly well
- Baochuan - the AI doesn’t know it can generate influence by parking their UU in CS territory.
- Iron chariot - the AI won’t be able to recognize the resource management ability, though foregoing the resource management CS bonus to spam a SR-free horseman is a perfectly legitimate strategy with the unit
So I would cound 6 out of 44 UUs that the AI could benefit from AI improvement in some way. Out of 44, which isn't too bad.