You've confused intended role of the unit with how its supposed to do it.
Presence of turret doesn't make jackson have different role then SU-85, both are intended to hardcounter everything that isn't an infantry and both are vulnerable to a flank(just one can attempt to defend itself, but it'll still die as P4 shoots faster and panther will outlast it without any effort).
Ah the mythical "intended role" that you use to wave away at any over/under performing unit. Wasn't the last time I checked, the "intended role" of a m36 was to be a 480 HP glass cannon?
And I dont know if you have actually used a tank with a turret (since you dont play the game) but having one absurdly adds to the role of a unit. Apart from the fact that a turret with a fast rotation speed (unlike the FF) makes flaking mildly pointless, having a turret adds this thing called maneuverability, useful when chasing tanks and performing flanks itself.
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It'll also lose to its weight in light AT vehicles.
Two Pumas vs a M36 may win, but the Jackson still outrages the puma, is as fast and has better accuracy. I don't think there is anything two pumas can do to chase down and kill a Jackson if inf or AT support is in reverse range. I dont think building pumas vs a Jackson is a sustainable trade to be making.