USP and SCAR are limited to line of sight, and really are not effective spacing tools unless there is limited cover.
I think it depends on where you place yourself. If you know the map well enough, you can force/lure people into charging straight towards you.
Also I disagree that they are the best guns for countering a Spas to the face, because you're going to be relying on the spas user to miss.
Well, if you get as close as possible and start moving and jumping around someone (don't need to bunnyhop, a few jumps is enough) it's actually quite challenging to for a SPAS user to hit you, because of the low RoF. There needs to be a bit of space to easily line up a OHKO shot.
That's just in my experience though. --- "God put obvious limitations on our intelligence, but none whatsoever on our stupidity". -Fr.John Corapi