If all else fails, the reality of ground assaults in EaW is that the defender can have however many buildings with their garrisons as well as 10 units - which means that even if it is incredibly costly, you can send assault after assault to wear down those defending units and eventually defeat the enemy.
The NR's weakness on the ground is that, on a unit to unit basis, most of the NR's ground units (particularly vehicles) are inferior to those of the IR in a head to head. A T3/4 company is weaker than a single AT-AT or an XR-85 company, and will lose in a straight up fight against them. The exception is the air units, particularly the V-Wings, which can be countered with AT-AAs or directed mass fire from other units (you specifically order a mass of ground units to shoot an air unit; this works well with most ground units unless you're being truly swarmed) - and none of those air units are garrison units, which means that buildings won't spawn them in battle, so any you encounter are built units that can be destroyed in "raid" attacks to diminish the enemy's defenses. Before you invade, probe droid the planet so you know exactly what it has on the ground to attack you - if it only has factories, you need anti-armor. If it only has barracks, you need anti-infantry. If it has both, you need a mix. If it also has garrison units, then you may want to use raid-style tactics to whittle them down - since the AI isn't big on holding its units back for a coordinated assault, if they've got a lot of fast air units, just dropping a couple AT-AAs with some stormies to guard them and blowing away the air units they send then retreating to re-invade with other units is a worthy strategy. Just remember that the IR's units excel at what they are designed to do, but are not that good at what they aren't - the NR's are more multi-purpose, so while the NR can get away with spamming a single unit type (a buttload of freerunners is nigh-unstoppable), the IR really does need those mixed forces supporting each other.
While ground battles can feel incredibly expensive, remember that it takes losing 6 AT-ATs to equal losing a Star Destroyer - a tough ground battle is almost always going to be less of an expense than a tough space battle. You can afford ground assaults with heavy losses far more than you can space assaults with heavy losses.