I always found this game very fair and balanced. Knowing and understanding the game world, build your guys up smart. Balance is key, not focusing in any one area for upgrades.
Also, doing this and playing smart makes the game a bit easier. Know when to rush, know when to sneak and crawl around. I found I used stealth far more than I rushed but every once in a while rushing two or three guards was the only option and using everything you have really helps. Keeping a gun great for indoors, and another that's better at distant shots. I used my pistol *a lot*. And that guy at the busted up gas station- bought a bunch of upgrades from him. That pistol, when upgraded, is *very* freakin' lethal.