Yeah, you've pretty much pinpointed it with your scatterscouts are midair and FaN scouts are on the ground. after playing a few rounds last night with the original setup i thought heck let's give it a damn shot.
so here comes a report on that. I also must note that the sandman is a tad overpowered when there's a skilled scout, or two scouts. you can just perform stunlocks on people, it really is annoying if you're on the other side, getting bonked, or as medic seeing your precious uber just getting wasted like that.
That aside it was all unlockables equipped and ready to go. I did notice (more than before since I had been scattergunning all night) that once you come close to someone they pretty much die if you have the FaN. no need to aim twice, it'll fire those rounds so fast anyway. combined with the Sandman (and jeez I missed that double jump when I started with SMan again) you can stun, run up and blast em to shreds. unless you forgot to reload... (happened to me.. yes...
the bonk!-drink gave me so much more survival. if you ran past a sentry first, got hurt badly but are trapped in a corner, drink up and run back safely. or annoy that sniper by him wasting his charged shot on a scout that's ''uber-charged''.
Most annoying thing that I pulled off (and maybe that's why the sandman is a bit Overpowered) is the following: uber heavy coming in, as he revved up he got stunned ''BONK!'', the ball rolled back to me so as the first stun wore off, ''BONK!'' again. and a third time to top it off, before the uber ran out. (from experience as heavy getting ubered and get it wasted like that, is most frustrating. Yes I have played heavy also today, but got Bonk-owned all the time)
Diminishing returns on the stun, has my vote!