Gunslinger is good, but Marines also have access to it and I'm not even sure it's better than headshot is for Technicians. It can be strong enough to go far depending on your luck with medkits/mods and good pistols, but overall I feel like it requires a lot more luck than playing Marine/Tech.
As you said, even using stealth to get into an optimal position leaves you taking massive amounts of chip damage. This is especially bad against anything with a rocket launcher, where unless you can 1-shot it, you're still going to take 30-50 damage killing a single enemy from an ideal position of cover.
Not as significant, but the Scout bonuses are really weak as well and tailored to doing speed runs and maybe some angel modes. On a regular game, you don't really care where the elevators and chests are because you're going to reveal the entire map anyway. Losing a weapon slot on top of that is especially rough since that almost forces you into taking gun hoarder for flexibility, basically starting with one less talent.
Using a good rocket launcher or shotgun to kill stuff off-screen helps with the constant chip damage, but it's a relatively frustrating way to play and still usually doesn't make up for the damage difference between Scouts and Marines/Technicians due to their talents. Recently, I've had more luck using this strategy with gun hoarder & juggler to make use of more (untalented) weapons than actually going for gunslinger, but it doesn't feel like the intended way to play a scout.