The biggest drawback with fighters, artillery and bombardment is that they simply act as front line units but fire from another position. These weapons should have a different effect in the battle in order to be important. As it stand neither of these things are important like they should. If you had complete air superiority, artillery and bombardment dominance it should multiply the effect of your other ground forces allot.
In the real world having superiority in artillery and air support is huge in a conventional conflict.
I also think that air units should just have been another ground unit that you could possibly have been operated from hangars directly from space as well as from the ground.
In general I think that ground combat would need a few more variables to become less one dimensional in nature. The current simple hit mechanic does not really encourage realistic army configurations, different types of units need to have different effect on combat as a whole as should the combination of units also have. In real life the sum of the part often are allot greater than the individual parts. In simple terms a tank is allot more powerful when supported with infantry and vice versa... Static and infantry units alone are pretty bad in a defensive role, you still need mo bile units, artillery and air superiority to do any type of effective defence, the same goes for offence as well of course.
I hope that Steve at some point make a second pass over the ground combat mechanics and make it a bit more dynamic and army design more enjoyable outside role-play.
