Part of me likes science-fiction. I've read an incredible amount of hard sci-fi over the years. Though I've gamed a lot of WH40K over the years, too. I haven't messed around with gaming hard sci-fi. The closest I've come is Full Thrust. I never ran a combined space and land campaign, either. My science-fiction tastes have grown "harder" recently. I'm liking more science in my science fiction. In fact, it's probably why I've had a hard time reading the GW Horus Heresy novel, Fear to Tread.
There is a group of gamers who love hard military sci-fi. They are in the minority. Most people play GW space marines. There are also legions of Star Wars and Star Trek fans. When I write science-fiction, I base it on what I know. I use the latest information about planets in our solar system or exo-planets in other star systems. When you confine your writing to reality, it makes the science-fiction "harder" or grittier.
Part of the problem is how you deal with the speed of light. If you assume there is no light barrier, then you can move military forces with the greatest of ease. The problem with that is the vastness of the universe. It took the Voyager space probes a dozen years to reach Neptune when they were launched in 1977. They were the fastest vehicles humanity ever launched. A space probe called Deep Horizons was launched back in the mid 2000's to go to Pluto. Pluto is five billion kilometers from earth. Deep Horizons won't reach its objective until 2015! It'll have taken nearly ten years traveling in deep space to get to Pluto. Unless there is a quantum leap in space travel technology like ion engines, it will take a human manned expedition 13 months just to reach Mars. The Moon is still 3 days away. And it is our closest neighbor at 225,000 mile distant. Mars is 40 million miles away from earth.
If you allow for ion engine or old fashioned massive drive technology to shorten your trips around Sol to weeks or months instead of years or decades, then you're confined to the borders of the solar system. To make the technological leap to interstellar travel requires several things. One, it requires Faster-Than-Light technology via a energy source like a naked singularity powered worm-hole. Or it requires suspended animation or cryogenic hibernation for sublight travel. The distances of our nearest neighbor, Alpha Centauri is over 4.3 light years. It literally takes light 4.3 years just to travel to the nearest star system from Sol. If we had the technology to reach 10% the speed of light (3.0 x10 6 km/sec), it would still take about 50 years to reach Alpha Centauri. Any military expedition to a nearby star system would have to be self-contained with its own marines, vehicles, and logistics.
Though it's theoretically possible to travel FTF with naked singularity technology. I highly doubt we've ever developed it because of the all the possible drawbacks when it comes to using it. I think future human interstellar colonization missions will be done with some sort of hyper-sleep technology and sublight space travel. The ideas of interstellar empires ruled a God-like emperor and his imperial space marines are mere fantasy. So much for gaming a "hard" sublight sci-fi setting!