The Jackson literally lacks any unique weakness (hurr durr it can't do shit against infantry!) which is a massive balance issue when you consider the stack of unique advantages it has.
Having 0 dps vs infantry actually is a weakness. The panther's coaxial and mounted MG can do lots of damage vs squads if the panther is close.
The Jackson also cannot self spot like the SU85, has no stun shot like firefly, stug or elefant and it has paper armor that gets penned by every anti tank projectile. Not to mention that a Jackson has 0 advantage over TDs like JP4 or SU85 on our beloved thin MOBA style 2-4 lane camp maps.