How so? It gets +160hp with veterancy which is a huge difference, a permanently useful vet 1 ability (Firefly has nothing, SU-85 has a meh ability, Jackson's forces a reload and costs munitions), and ultimately higher, it gets +38% accuracy while the others get +30%, and all other buffs (reload, mobility) are roughly the same. On top of that, most of its basic stats (target size, reload, and accuracy) are better than the other TDs in the first place (sole exception is that Firefly has equal vet 0 accuracy and slightly better with the tank commander).
The vet 5 ambush bonuses are still decent. Even if the extra damage doesn't do anything, it still gets better accuracy and penetration for every first shot.
The only thing it lacks is a penetration bonus, but again, that is on purpose.
Beyond vet 5 being a bit lackluster, there is absolutely nothing about its veterancy to complain about. I can only agree with:
I almost always use Jagdpanzer IVs, they are fantastic to play with, and a real pain in the ass to deal with when playing against them. It is a really good unit.
vet 5 bonus are not decent, when other tanks get them permanent and not only for the first shoot out of stealth, the increase damage is almost useless
and u are exactly proving my point, as u said ur self
"it gets +38% accuracy while the others get +30%, and all other buffs (reload, mobility) are roughly the same."
it has no business having vet 5 at double the exp requirements of other TD