XP used to be directly tied to cost.
Most patches that change the cost of unit did not change the XP values accordingly while some units had the XP changed so currently XP value are mess.
XP values of entities
XP value of entity itself is enough to determine only in wipes. In the case that squad survives one has to take into the total damage dealt multiplied XP so a weapon firing on conscripts squad probably has to deal 1.5X damage to gain the same veterancy as firing on grenadier squad.
But what I did not claim that ML-20 vet faster , I simply pointed out that I see no reason why LeFH would vet faster.
From my experience OKW trucks are a prime target for ML-20/120mm mortar since the are big and have high XP value and if one want to gain veterancy one should aim for them.
You didn't say that the LeFH vetted faster than the ML20, but you pointed out that Soviet models give less XP without the context of larger Soviet squads. It doesn't matter how much XP the single model gives, but how much a shot gives. With Soviet squads being 1.5x times larger than Ostheer ones, it makes sense that Ostheer squads need to yield 1.5x times more XP, because larger squads have a higher chance of being hit due to covering a larger area and you hit more models in general. How this plays out in detail from shots hitting in the middle of the squad to only clipping some models with their far AoE is difficult to answer. Maybe MMX knows more, but I remember him saying that TTK (in the case of infantry vs heavy tanks) does not depend on the squad size. But again, the only things I wanted to point out is:
1. Soviet infantry yielding less XP is a necessity due to larger squad sizes.
2. USF and UKF exist, which yield fairly high XP values compared to Soviets.
As a last point, there are also Soviet Penals builds which combine both high cost and large squad size.
I am not sure if that hold much water. One the most popular soviet commanders is Soviet Combined Arms Army that combines ml-20/reckon/IL-2 bombing that could clear LeFH.
I would guess that is Soviet did/do build less ml-20s is related to the fact that there are more option available to allies teams for arty like B4,Priest, Sexton, calliope, Land mattress or the fact that soviet have access to super heavies so they might choose to go for them instead.
On the other hand Axis are in need of counters to 120mm/Pak howizter/emplacements.
In any case is rather easy to delete howitzer with off maps which UKF/USF officer can easily reveal so I don't that usage speaks volumes about value.
On the other hand I do agree that removing counter barrage was a (probably justified) nerf to LeFH.
The dive bomb has been removed from Jager Armor specifically after the community requested it to be, because artillery to bleed out your opponent was one of the few counters to Elefant builds.
Sextons and Priests surely are a contributing factor. But if you look at the commanders in Ostheer and OKW back in March-June and the fact that your ML20 when in doubt would lose the shoot out (at that time especially with CB), it is not surprising that ML20s are not popular.
At least in the current build, Axis have a very good counter to emplacements and infantry based arty with OKW's Stuka.