Well alright let's give this thread a go, never thought I'd be discussing JP4 on here. 3v3 perspective mainly.
Many points have already been addressed. It's not like it has 230 armour and
a turret. By the time it arrives your main opponents are meds (range 40), Comets (45) or enemy TDs (60). Two of which can't shoot you unless you're careless (or using JP4 on the edge of the front) and the others will punch through the armour like butter. Comet will consistently pen too. If they charge you with a med, you better hope for some armor and fast unless you can get the gun pointing at them just right whilst reversing, free shots on you all the while you don't. The closest analogue is the old SU-85 with lower pen and better fire rate. But you get 45 sight range instead of Focused Sight, meaning a med is already in shooting range by the time you see it.
I personally use this vehicle very heavily but it's a rarity/very uncommon to see in other OKW players' hands. I've actually been mocked by a top 15 Axis player for using it. A very high skill threshold imho and thus many ppl prefer Panthers. A pair of them can scare away Jacksons/Fireflies (to a lesser degree SU-85) or even kill them outright, but it's dangerous. If they decide to hard commit to a slug out, it's usually because they have something else in the picture.
Vet III to vet III, your size doesn't really matter (unless FoW) and everyones' TDs gets hits. Imho it's exchanged pen/dmg vet bonuses for (hp) and later arriving acc/rel.
tl;dr The unit is fine imho. Yes skill & improved tactics can mitigate its weaknesses/make its strengths deadly etc. but that's not really saying anything.
EDIT: Does anyone even use the vet 1 ability? Is it just me in this place? 300 games and I've seen it maybe 3 or 4 times.
Didn't we already nerfed Jackson's armor because P4 "sometimes" can't penetrate at long range?
Jackson has
a turret, 6.5 move spd (to pz iv 6.3) and a ludicrous 3 acc.