I have prepared a brief table from the encyclopedia to help you compare and contrast the units available, limiting it to the 1941-45 period. the rate of fire (rounds) data is not entirely accurate but provides a baseline for further investigation
Unit Name Cost Rounds Cost/Rnd Warhead Pen Avail
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US 4.5" rocket__ 70 32 2.2 7 42 Jan 44
US Calliope___ 88 32 2.75 7 32 Aug 44
US Truck Rocket__ 69 16 4.3 7 42 Apr 45
UK Land Mattress 80 48 1.6 9 48 Oct 44
GE Nebelwerfer_ 53 12 4.4 13 51 Jun 41
GE Wulfger 4.1__ 46 10 4.6 10 42 Jun 41
GE PzW-52___ 51 15 3.4 10 42 Jan 43
Jap 20cm Rkt___ 47 6 7.8 13 64 Jun 44
Rus BM-8 Katyusha 47 24 1.96 5 32 Sep 41
Rus BM-13 ____ 51 16 3.19 10 48 Jun 44
Rus BM-31___ 76 12 6.3 17 98 Jul 41
note the large number of rounds fired in many cases and compare this with the much smaller number of rounds fired by large caliber howitzers, etc and you will get some idea of why i love rockets. The same Off-Board units generally fire more rounds than on-board units.
Try out rocket artillery and enjoy the wailing of your enemy.

PS sorry about the inability of the msg board to correctly format this table