Why is why I compared it to the ZiS gun and pointed out you don't want to rely on ZiS guns to kill a King Tiger.
The issue with the Rackten in facing the IS2 the most however is the fact it's really easy to 1 shot due to the high AoE of the IS2 and the extremely close grouping of the crew.
The JPIV's issue is mostly just pathing and being overwhelmed, it has really, really bad turning speed meaning that you need to micro the fuck out of it to keep it in the action.
you said the raketen had low pen, it does not; heavy tanks have high armour. of course dealing with heavy AI/AT tanks with AT guns is tough, that was one of the problems with USF before the jackson buff. OKW has plenty of options though. they have raketens and schrecks (as mentioned, not very good against heavy tanks) for weak options and the puma, jpIV, V, and VI B for stronger, less bleeding options, with the Pak 43 as a doctrinal choice. obviously, mileage may vary with the jpIV and puma but they can still work. OKH is about the same as OKW, just with different options. all AT guns get one shot quite easily by heavy tanks and the raketen's range weakness is offset by the easy availability.
and seriously, i just used the jpIV in a game and the turning is fine. i got it to vet 2 before i got too careless and let it engage an SU-85 with AT gun support while forgetting to move my two schrecks up. i got 13k damage on just it. the mobility isn't much of an issue if you use the jpIV at its intended range. obviously, there are situations where casement TDs don't work and the puma probably won't work well in that situation either; nor will the stug or su-85.