I wouldn't really call the style "anime". It's a style that has more in common with animation or comicbook art than illustration, though, whereas the wizard portraits are much more detailed and stylized. Buildings and mechanical-type units (cannons, ships, golems, etc) border on cartoony, and really look like the least impressive of Thea artwork. The other unit portraits are hit-and-miss; some cleave closer to the wizards, and some cleave towards the animated style. It varies by race, to a certain degree; Goblins are better than ships & buildings, but are very Thea (I don't think this is a coincidence, either); Gnolls, Lizardmen, Orcs, Beastmen, and Trolls are a bit better but also a bit flat; Dwarves are generally decent outside of the mechanical units, as are Halflings, Draconians, and (surprisingly) Klackons; all sorts of humans and elves are probably the best of the lot. Summoned units are all over the place, but tend towards the lower, more-comic-bookish end of thing. They're generally a bit better than Age of Wonders 1, but they still invite comparison to those...
MuHa has had issues with inconsistent art quality in the past. They did an update to Thea 1 at one point that added a bunch of character portraits that they ended up rolling back in a later update after they received complaints about quality. I've always gotten the impression that they're a small developer on a tight budget, and they try to cut corners on art and writing from time to time - not consistently, but periodically. If they could have managed the same level of art for all the units that they did for the wizards, I'd have been very happy with it, but that's probably outside their budget.