I disaggree on both first if i am not mistaken white phoshorus is a vet 1 ability which means you have to9 get there first and you already admitted that the barrage is lackluster ,not to say anything about its normal attack which is abyssmal , on top of that it costs a whooping 40 fuel , in a faction where you need to get to the major tier as soon as possible and the efficiency and timing of the m20 utility can decide whole games . You see then that from a strategy standpoint the mortar ht is a mistake no matter the circumstances.
On the pwerfer first of all you fail to see the bigger picture , namely that it doesnt pack enough punch to equalise the effect of a kv8 , if you go t4 you usually want to get a panther very soon for obvious reasons , second if you drive the werfer that close to enemy lines you risk it being killed by an atg you didnt know was there or a mine or some other random event which is precisely what you want to avoid with the pwerfer , otherwise you could have used the much cheaper and by that time possibly vetted axis mortar ht tah actually performs reliably
you are mistaken, as the white phos does not require vet. the timed charge barrage requires vet.
equalise the effect of a kv8? what does that even mean? the primary use of the werfer is to clear out late game defensive positions such as machineguns and anti tank guns. a werfer can quickly clear an area where as a mortar you have to wait quite awhile, and in the late game where VPs matter the ability to clear the area for infantry and tanks is quite useful. of course, use common sense on how close you can get your panzerwerfer, dont run it into the open with no forward units to spot for threats, thats just bad play.
the mortar ht is quite reliable and comes out early, and has access to the incid barrage, but it is doctrinal.