Then you lose the game, just as relic intended
i don't know if you played DoW2 but bC once said that if you (as eldar) let SM get to t3 as force commander and shit out lightning claw terminators you deserved to lose.
Yet I never hear complaints of the Panther being unreliable against Soviet heavy armor despite extremely similar chances to actually penetrate enemy heavy armor.
i haven't used Vs against IS-2s in a LONG time (for various reasons) but what i used to do is either snipe at them from outside the IS-2's range, hoping to get damage and force a retreat or drive in as close as possible and circle strafe them. trading blows at mid/long range was a HUGE gamble that i wasn't interested in taking, especially if there was other soviet AT around.
as for the old IS-2, a huge part of the problem with it was that if it missed (it didn't have pen problems that i remember) you were waiting a long time to do something again. it was still an RNG gun (and vicious against infantry).
the jackson reload is a little (by a second or so) on the long side for an AT gun and it has poor pen. additionally, as others have said, it's the best non-doctrinal option the US has; all the others are even more RNG based. US doesn't have any good delaying options either; they can use tank traps (i don't think they're crushable) or M20 mines. the first is time intensive, easily disrupted, only works when for areas you want to hold at, and the traps are destroyable. M20 mines are very strong but also expensive and require T1 with an M20 and that the M20 stays alive to plant the mines and that the enemy runs them over.
When i play as US i tend to run into issues when the enemy attacks with large numbers of heavy tanks because jacksons just end up overwhelmed. ironically, jacksons preform best on open maps but the US defensive options work best on small maps. this isn't unique to US but they do have limited options, as do non-doctrinal soviets. axis has an easier time because their options are more rounded.