I've been very quiet about this statement for a while, as I didn't really have that much modern experience dealing with the Z-6. However, after being HVYL for a week and having a lot of time to get accustomed to the Z-6 in that week, I feel like I can throw in my input.
First off, the Z-6 isn't meant to be popular or very good. While it's supposed to be a heavy weapon, the Canon Wiki Page says enough.
"It was extremely heavy and required its user to stand still to be used with precision. On top of its weight, troopers equipped with the Z-6 would also have to carry its energy cartridges. Replacing the weapon's cartridges took multiple minutes, and would force its user to leave the battlefield during that time. Merr-Son engineers attempted to find solutions to these problems, but none of them were satisfactory, which tainted the Z-6's popularity."
Even in lore standards, the Z-6 was not the best. If that's not enough, the Legends Page only adds that it's supposed to not have good recoil.
"Despite its impressive firepower, the weapon had several notable limitations, specifically the high rate of energy pack consumption, significant weight, and powerful recoil."
After using the Z-6 for a bit, I can say that its spread it pretty trash, as well as a few other things. Though I have been able to mow down enemies with it. I'll admit that sometimes I wish it was better than weapons like the Clone Minigun, it's supposed to be less popular lore wise.
Now granted, I do believe the Z-6 should be buffed. I feel that compared to the other class weapons, the Z-6 is the worst of them all. The recoil is lore accurate, but in game retrospect, miniguns tend to have better spread and accuracy when crouched.