The Jackson's weakness is the same as the Firefly's - infantry.
Not really - the Jackson can run from infantry at will since its acceleration is 3, same as the infantry's. The Firefly can't, it is slow to accelerate to speed (1.6) which means it cannot disengage so easily and can be caught for a fatal Shreck volley.
If a US player brings out a Jackson that's a Sherman they didn't build and therefore an increase to infantry-killing power they didn't get. There's very little a Jackson can do against a sidegraded Panzergrenadier squad or an anti-tank gun except retreat.
The problem with that is that the USF does not really need that Sherman. It already has large Riflemen blobs by that stage of the game which can pretty much crush OST's infantry on their own
without any help required from a Sherman.
It's a fundamental design issue with the USF faction. Its one-dimensional - USF is an infantry spammer early in the game, which costs a lot of manpower, but no fuel. Once there is enough fuel, it just needs to spam TDs to protect its infantry while it is steamrolling Axis infantry from any Axis tanks; the same infantry also takes care of any ATGs or Shrecked Pgrens very quickly that
could threaten the Jackson. Actually the only
In contrast OST absolutely MUST build something for fuel early in the game because their infantry sucks and their weapon teams are too inflexible; this just delays their tanks and giving USF infantry even more time to operate unmolested; though luck for OST that their early light vehicles are also pretty poor and absolutely incapable of forcing the USF to react to them, i.e. divert in any way from building RM and then Jackson.