At the Beam End of the Universe
I have played quite a few Sci-Fi games in the past - from Laserburn to 40k. Great fun - especially the 'bucket of dice' approach of the latter. Not to be taken too seriously - after all it's only disguised colonial warfare. Ah! but that's where the niggle is.
Imperial Commanders, Space Marines and their like are vastly over-powered and over-armoured. The 'natives' have to rely on numbers and fieldcraft. I prefer more evenly matched opponents.
A scamper back through history shows that where extreme conditions prevailed the 'Imperials' didn't do so well (Viet Nam, Afghanistan, etc). Science fiction is full of such worlds - and you can invent even more.
So Why would anyone (or anything) fight on these worlds? Who get's assigned to remote places and hopeless causes. Well here's a few suggestions:
Those who have fallen foul of the authorities for political, religious or family reasons (think Romulans or Klingons).
Those who have proven to be less than enthusiastic when it comes to fighting (cowards, conscientious objecters, pressed men, etc).
Those who have reason to run away (criminals, loosers in love, persecuted minorities).
I'm sure you can think of others.
Example One: Jungle world
Conditions - High levels of moisture (in the air and under foot); lush and dense foliage; a breading ground for bacteria and fungi.
Force One - a small group of Imperials charged with keeping this sector free of enemy incursions - and keeping the natives in order. Advanced equipment and armour soon become useless because of voracious microbes in the atmosphere. Power cells the same. Armour is improvised from the shells of various sea-going creatures. Weapons are simple slug-guns using explosive propulsion. Swords and spears are not unknown. Chain saws are just for show.
Force Two - the natives - at home in the terrain. Armed with powerful bows and exotic melee weapons. Semi-aquatic. Small in number.
Force Three - orcs from the wild fringes of space. Settled, many generations ago, in areas that the natives don't use. Apart from trading they ignored each other until the Imperials turned up.
Rely on melee weapons and shields. More intelligent than their mainstream relatives. Small in number.
Situation - High Command expects even these third-rate troopers to do their duty. Being remote from the planet means they know nothing of the prevailing conditions nor care. On the ground it's improvise, adapt and survive.
The locals see the Imperials as invaders and exploiters.