So far what nations look strong... and weak?

[PvK]

I had no problems using Ulm (Iron Faith) up to turn 40. They certainly had no problem stomping Mictlan, often because Mictlan armies would rout when their slaves started dying. (An idea for an advantage to add to slaves would be to make them not cause morale checks when they die. At least, the AI might be programmed to put them in their own squads to help, although I think it may already be doing that.) I suppose Mictlan probably does much better under human control, and maybe if it does more blood summons.

Meanwhile, as I mentioned elsewhere, my Arco hasn't been doing well against AI Marignon's crossbows, HvInf and HvCav.

These may have a lot to do with my own experience level and play style, though.

Iron Faith Black Priests do get a random magic pick, though, and since they're holy units, they're cheaper than say an Arco Astrologer. The forge bonus can be used to cheaply make items which increase magic power, eventually.

PvK