Well the hills aren't "balanced" as well.
Zero-K and Spring maps in general are very varied and most games plays well on many maps.
Since there isn't any strict map formula like in Starcraft and players have so many factory choices it is hard to say what makes a map unbalanced since there are too many factors to take into account. An uneven mex distribution might be offset by different terrain advantages from hills or plains, cliffs or smooth ledges.
In Starcraft, buildorders need to be adjusted to the map. If there is asymmetry on the map it gets complicated as you'd have to practice on different starting positions to get a good buildorder and be able to create a tight wall to prevent Zergling run-byies.
ZK doesn't have any strict buildorders like that so you don't really have to practice on different maps to get a good buildorder so asymmetry isn't as bad in ZK.
I like asymmetric maps.