ATCS is unbalanced. Symmetrical does not mean balanced, there are hundreds of other factors like the distance between bases, or the number of alternate routes, holes, and hiding spots, accessories like safe ledges for Dragoons to pounce onto, and even the height of the average ceiling or smoothness of the average wall.
Anyway, turns out humans have it their way on atcs because of the very short refuelling trips and very simple map construction. No eggspam here.
edit - as for why tremor is so human, it's generally the lack of decent alien base locations and the inconvenience of the unjumpable walls I guess.