Now i am trying to figure out he OHQ units, i see the great advantage of stratagems like infantry getting 50% defense, but combined arms? Not so much . For exemple a medusa type planet was covered in woods , i assume you should use only infantries in such terrains , or am i wrong ? The few plains are covered by light tanks. But i see no reason to build combined arms or mechanized infantries for that scenario. When does those combined formations shine ?
Combined arms if You mean assault infantry (inf+tanks) are not great, but if the whole formation is motorized, or better, mechanized, they become extremely useful. The problem with tanks is they cost a whole lot and tend to take casualties when being attacked. Mixing them up with infantry helps their survivability and spreads the heavy firepower of Your army across more units, while keeping the cost down and numbers up. Additionally such formations do better in mixed terrain (where You have mostly plains, but with scattered forests and hills).
That said, on heavily forested medusa worlds I would go full on infantry and artillery, with some additional recon and eventually walkers.
