I gave you the information that is relevant for AT guns.
AoE Pen is mostly irrelevant and the difference is 5.
You NEVER take in isolation RELOAD values, because it's only 1 component of the shooting cycle. IIRC it should be Ready aim + Fire aim + Wind up >> Shoot >> Wind down + Cooldown + Reload
Which is why i listed ROF and TTS.
Vertical rotation is also irrelevant.
The differences are minimal. The fact that the Zis have access to vet 0 barrage and +2 crew offsets the 1s difference in Rof, the 2 rotation speed difference (which was added to help the faction against light vehicles) and the 10 difference in penetration.
To make it complete, vet wise:
https://www.coh2.org/guides/29892/the-company-of-heroes-2-veterancy-guide#1220
Zis:
Unlocks the "Tracking" ability.
+30% rotation speed.
+30% reload speed.
+20% reload speed.
+30% penetration.
+6.25% range of the "Light Artillery Barrage" ability.
Pak40:
Unlocks the "Target Weak Point" ability.
+30% rotation speed.
+30% reload speed.
+10% reload speed.
+30% penetration.
Pounder:
Unlocks the "Rapid Maneuvers" ability.
+100% rotation speed.
+30% reload speed.
+30% accuracy.
+30% penetration.
Yes, I totally get your point. But these seemingly irrelevant drawbacks are usually game-changing. I have been testing out for dozens of games(4v4 rank around 70) by asking my friends to para dropping me 6 pdrs, and Zis usually ditch out half the damage despite being presenting twice longer the time. Just taking the last game I just played, for example, the single pak 40 I captured for only 10 mins, have dealt the same damage as the Zis, which have been on the field for nearly 40 mins.
Here is my point, yes, some people do purposely build Zis for its utility. But, these utilities will phase off dramatically as the game proceeds and usually do not affect whatsoever in the late game. I don't expect zis to be as good as pak or 6 pdr, which it shouldn't.