Remember when the howitzer was so OP that it took this to nerf it... And it was still a viable gun.
Field Gun has a decreased turn speed, rate of fire, turn radius, as well as an increased reload time and slight drop.
And that was before a subsequent nerf to its damage and drop.
Nothing has been done to even slightly reverse those changes since. Therefore the current merc is a result of massive amount of nerfing.
So next time somebody complains about that gun, I'd just like to say:
We all have it easy right now.