Im not lumping them together. You are, and you say jackson should be nerfed because its not su85 or firefly. You mention all the things su85 and firefly cant do and conclude thats the reason jackson should be nerfed. Jackson is different unit in different faction with different playstyle and has different set of (dis)advantages.
What is the reason jackson should be nerfed other than being different unit from other TDs and whining from axis mains?
I don't think the Jackson needs to be nerfed. I think the Jackson was buffed to its current state for a reason: the 480 HP Jackson just didn't cut it in long range slugging matches, and the rest of USF's AT lineup is so ridiculously geared towards anti-light vehicle duty that it was just too easy to overrun.
However, if there's a way to make the Jackson vulnerable to close range flanking without crippling it in long-range slugging matches, it'd be good for strategic diversity within the USF faction. The 76mm and the Easy Eight are always going to struggle to find their place when the Jackson can do their job and usually do it better.