ok, so after doing some analysis myself i'd conclude the hm-38 is mostly fine. its barrage has lower scatter than the american m1 and the grw-34 and is only slightly worse than the pm-41. in addition, due to the superior aoe profile it will vastly out-damage the other mortars vs infantry at all ranges.
dps-wise it is also better than any other traditional mortar in the game despite the lower rate of fire (time between shots is 8.6, 7.1, 5.6 and 5.6 for the HM-38, PM-41, GRW-34 and M1, respectively).
the only indirect fire team weapon that is able to out-dps the 120mm mortar at 80 range is - unsurprisingly - the pack howitzer.
i'll post some more in-depth analysis illustrating my point later
As much as I like your scatter data sheet,the issue here is that the barrage of the heavy mortar is very useless.
The time between shots is so long that you can only use it on structures. If you use it on enemy infantry, your opponent has plenty of time to move away after the first shot. Even team weapons can be moved decently well. And then you have to hope that the first shot lands very well, otherwise your opponent might not get too much damage at all for such an expensive unit. Against garrisons the issue remains the same. The third option to use it against static buildings like bunkers is also bad, because you could do more damage with the normal mortar. Additionally the low ROF makes it vulnerable to rocket artillery in the late game because it sits at the same spot for 40 seconds. The only virtue of the longer range, slow firing barrage is that you cannot be counter mortared and have a long area denial.
Yes, it has a bit more range, but only for the barrage and as I explained the barrage is quite shitty in most scenarios. The autofire on the unit might be fine, but the barrage is not. And this is exactly what balance team wanted to move away from and redesign mortars as support units that need active management. Ever since that patch, the 120mm became very bad, because it was hit by the damage nerf harder than all other mortars but cannot really support better than the standard SOV mortar. It's probably even worse due to the lack of flare at vet0.
And one last thing which I am not sure about though: I got the impression that it is more clunky to use. It needs to turn more often and/or at least has longer setup times.
All in all I don't feel that the 120mm is in a good position. You pay more to get a mix of slightly better performance in some circumstances but also much worse performance in other aspects. There is no real benefit and no real situatuon where it is consistently better than the normal mortar, especially conaiderung the quite hefty price.