It depends which matchup we are talking about. USF and UKF really don't shine in the indirect fire department, unless you take the same 1-2 commanders always. Soviets on the other side have a very useful nondoctrinal mortar, strong nondoctrinal rocket artillery and a bunch of commanders with come with one of two doctrinal howitzers or the doctrinal 120mm mortar.
USF/UKF nondoctrinal -> definitely axis nondoctrinal indirect fire superiority
with commander choices this may look different
Well, since it was context of mortars.
Soviet PM-41 is pretty much on pair with ost mortar. It all comes to the fact who can land few hits first, both have chanses to do it with a first shot or do nothing. Both need to be very close to the target for maximal effect. Soviets can use flares to boost accuracy, Ost have fast firing mortar.
USF. Fast firing but low range mortar. It hits quite good, but the main disadvantage is range, you really need to get it close to the target. On open maps, yes its weaker then ost mortar, on maps with sight blockers, where you can get it closer, it performs some what on pair.
UKF. Commander mortar is a copy of USF one I belive. Without commander, yes, UKF struggle a bit vs mortars early, but you need to survive like first 3-4 mins to get your mortar pit, which shut downs Ost mortar play.
And OKW IGs sucks ass, when it comes to countering other inderects if its not mortar pit.
If we look at indirects over-all, then the only advantage axis have over allies - the fact that UKF\USF dont have stock rocket arty.
Poit was that 120mm mortar being at 2CP is a hard counter to ost heavy support weapons oppenings, which is true. But along side being hard counter to one particular cancerous strategy, it also just as effective against pretty much any other strategy.