This is a case of observer bias. Jackson has about 15% chance to bounce a max range shot of a P4. Not sure what you want with the caliber of weapons here though. The game works completely differently and is far from realistic.
Also the Jackson should not get touched by Panzerschrecks unless you really misjudged the situation, get caught by surprise during a push of yours (but then there is normally no follow up for the Axis player). The Jackson has 60 range, Schrecks have (don't know exactly) 30/35 (someone please confirm) and need a bit of setup time. If your backline unit gets into knife range with Panzerschrecks that it deserves to be punished. Also in a tank rush it can still drive away unless you're overmicroed heavily or positioned it in front of a path blocker. It does not get deleted much quicker than a similarly prices OKW P4 vs an Allied composition.
The Jackson has no drawback apart from a slightly underwhelming armor for the price. The only reason why it's strong is that it has to be that way due to the design of the whole USF faction. Every other unit in the game has a weakness, either be it a strong counter or a clearly underperforming stat or feature (like casemates) that can be exploited. A Jackson however is usually lost by either the USF player making a mistake or being heavily outplayed which would probably kill every other tank in the game as well.
As you said yourself, 15% at max range. I want to believe that the bounce rate is reduced as the P4 gets closer. As for the caliber statement, I replied to the other guy that said that vehicles lighter than P4 don't always penetrate the Jackson. It has some armor, not paper armor. Moving on.
3-4 Shrek shots are enough to take down a Jackson. The problem is that the games I play are a blob shitshow from both sides. Enemy brings armor, I respond with Jacksons, he then brings AT equipped squads and I fall back while holding him with my infantry. Still not enough to stop them sometimes.
Yeah, the Jackson is fast, but if you have 1-2 and the enemy rushes you with 2-3 P4s and is focused on taking them down, he won't back down until he destroys the Jacksons. Plus, if you find it difficult to destroy a Jackson with a P4, you have alternatives. And if you don't, then use P4s and go up close and personal. If you can't take it down from far away, get close. Jackson is nothing more than a sniper. Good from afar, mediocre if you get close. This is how I see it.
And a solution would be to make the M10 Wolverine non doctrinal. Seriously, I wouldn't mind having it as a stock unit instead of the Jackson. It costs nothing, you can spam it and it's seriously perfect in every aspect. You don't even have to change it. Spam it, roll over the enemy, replace casualties. What's 80 fuel lost?