The assault class does wreak havoc...until it turns the corner and gets stopped dead in its tracks by a Support holding a Sickle. So yeah, it really doesn't. A strafing assault with sprint on isn't even close enough to being too fast to be untargetable, he's not going to be able to run into a room and clear it with a Sickle when turrets are sitting across the room, and he sure as hell won't be able to equip stun/sprint/Sickle/grenade launcher all at the same time.
What a Sickle Assault is going to be able to do, is to turn the corner quicker than a Sickle Support can, get a quick pick off (if someone's close enough), quickly retreats because the other team + turrets are about to light him up during pump-action, and hopefully gets a couple more because sprint has a short duration.
Support weapons not effective at long range (which, relatively speaking, is really mid-range, with sniper rifles being in the long range category exclusively)? The class' got Pulver, Purger, and Voltage; I'm sorry, but that's just flat-out wrong. The support does excel at defending a point, but it's through area-of-denial methods (covering/suppression fire, area of effect, etc)- not with a 1-shot 1-kill weapon.
I should clarify that I made a mistake, I meant to suggest giving the Pulver to Scout. It's a middle-of-the-pack AR that I really liked as part of an all-purpose Support loadout. You've also conveniently ignored me mentioning side-arms in stealth as an alternative.
None of these hypothetical changes I've proposed is going to single-handedly "ruin" the game, you're using multiple points to collectively argue against singular changes when they're meant to individually address each skill/weapons' shortcomings. Example: you using a) weapons in Cloak and b) AR on Scout
"I'm not saying that the classes are perfectly balanced" is probably the understatement of the year pertaining to Shadow Fall. As it stands now the games are won and lost on the back of Supports. Guerrilla games made a conscious decision to trim the classes and distribute the weapons/abilities to the remaining ones. They also designed the maps. It's pretty apparent GG's made several wrong decisions that further compounds the imbalance in-game we have right now.