The issue with 120mm is that you have no way of countering it and 120mm counters leIGs, HMGs, AT guns, infantry fighting in cover, static buildings like tech building for OKW and so on, while OKW has to go mechanized to at least pressure the 120mm, because you need to kill 4 models to at least decrew that thing. With OST it is all bad, because Werfer is an RNG cannon and is very inconsistent if you fire further then 60..70 ish meters, so it is not even worth for OST to try doing anything against it, unless you go LeFH, which is a less then desirable solution at least for me.
I think 120mm is so good that the single mortar might outperform 2 leIG
The 120 has a slower rof bigger scatter. It takes quite long to vet up imo, cuz its rof is low and acccuracy it misses a lot more esp at max range where people tend to use it. Its also 360mp has inflated popcap afaik to limit spam potential, maybe they changed that.
People just get intimidated by the big oompf sound it makes, while it only has a bit bigger aoe and quite a bit more range.
A single 120 is not as good as 2 leigs. Unlike the 120 the leig is quite accurate, it hits its targets more often.