Having played all 3 on a EV map while Attacker, I definitely feel much more “aware” when repairing EV, if given the time to lay out my deployable(s) when running Bush or Proxy.
As Bushwhacker, I got fair amount of HP to repair even under slight gunfire, and can stick turret so that it’ll notify me if baddie is trying to get me from that direction, hoping my teammates can cover the rest or at least give a hint by gunfire / death messages if I should make a run for it. As Proxy same applies but with Mines, since even if enemy simply shoots the mine instead of tripping to it, even half-deaf yours truly can hear that magnificent blast a kilometer away.
The escorting part? Yea, moving turret can be tedious and it needs a tad more gamesense to do so efficently. This is where Bush’s own strenght shines if that plan fails; he has decent HP pool to speed- ratio, access to some very versatile weaponary and pretty sweet offensive augments (looking at Tough etc.) to get stuff done by himself too. Proxy, as you said, has speed, speed and more speed. Dem double SMG’s are an absolute beast. Mines can also be used as clumsy, yet highly dangerous grenades too for more offensive approach.
Fletcher is as said here, good all-around guy. His stickies need more active approach, as they can’t be used with place-and-forget, so not as reliable for covering your own buttocks while introducing that EV to some blowtorch. But in case I somehow else notice enemy trying to open me another ventilation shaft with high-velocity lead, I feel like being able to react faster as Fletcha, although I do miss Bush’s KEK in those situations. Our Nigerian prince feels more comfortable to escort the EV at least in terms of being able to use my ability to it’s full extent.
Good enough reasons to pick w/e of the current Engies for Attacking? I think it falls down more on how well abilities and guns of the merc fit the player, rather than how merc initially seems to be played. Gut-feel is not a bad thing, but it never tells the whole story.