You focus on a relatively minor difference and handwave away the actual one, that looks a bit silly. Mjollnir's main advantage is energy being better against shields, not accuracy.
In that vein Heph has ~2.7x the base efficiency of Mjollnir against armor (including OP diff in vents). Yes, it doesn't have as much accuracy/hit strength, but it has enough to be acceptable.
I thought I did not need to spell Mjolnir's better-against-shields out (which I almost did but ultimately decided not to on the last post), but yes, Mjolnir does that, and it better for the extra flux cost. Without Rangefinder, slots to fit weapons with enough range are generally too limited. (And, yes, you pointed out slots.)
With the elite PD range bonus it actually has a longer range than 900 kinetics, and skill+armored mounts give it noticeable damage in standoffs though
Okay, after testing ePD+Devastator, having ePD and enough recoil reduction puts enough shots close to competitors' ranges to work as you say. Kind of feels like a variant HE Volatile Particle Driver. Without ePD, Devastator does not pump enough shots at long enough ranges to compete with other heavy HE.
When I use Hellbore, Hephaestus, or Mjolnir for anti-armor, I need the range, often to match complimenting Mark IX or range-boosted light kinetics. If short range was not a concern, then of course Devastator without ePD would be an option.