The way the ground mechanics look to be programmed now, on terrain that favours vehicles (desert, barren, ocean, basically anything largely flat and featureless) combat is likely to be short and favour the attacker because orbital fire support is just that much easier and the defense can't stack its defensive bonuses so well. On terrain that favours infantry (mountainous, forests, jungles, or worse, mountainous forests and jungles) the combat greatly favours the defender because orbital fire support is nearly useless while defensive bonuses can be stacked to ungodly heights. It also allows the defender to build up massive supply stockpiles, far greater than the attacker can bring in unless they've effectively won already, and possibly allow for a reversal of the space war that allows the defenders to receive reinforcements.
This doesn't mean that the defender will lose in terrains that favour the attacker or the attacker will lose in terrains that favour the defender. It just means you need to keep in mind what sort of units and how many units you deploy in a given circumstance. It's entirely possible for example for an attacker to not actually enter an aggressive posture on a jungle world and just stack their own fortifications as high as possible just as the defender is doing and slowly grind away at the defender as a result. I'd strongly advise this unless you have a definite tech advantage, or you can bring in large numbers of ground forces and keep them in supply.
