Its a double standard discussion here.
Why? Just because both Stuka and Katyusha both have powerful barrages it does not in any way, shape or form imply that they are EQUALLY overpowered and what applies to one, should necessarily apply to the other. This is a total non-argument. Please refer to
this post of mine for a bit of details.
With cooldown time included, and adding a bit to both sides to get into position and fire, you should actually be comparing somewhere around 24 to 28 of Kat's rockets to 6 Stuka's. Please everyone tell me now if that is ok in your book. (Deliberately not talking about Werfer here).
Kat saturates a large area, Stuka shoots in a precise line. As much as the line thing can be a blessing it depends on the enemy arranging its weapon teams and infantry in a line AND not moving too much once the rockets are fired. Compare with Kat's circle of doom. The AoE on Katyusha rockets is good enough that it damages everything in the area its rockets fall in (no unit in this area can reasonably escape undamaged or with small damage), which makes the effective area of denial a Katyusha can achieve much greater and longer than the Stuka, even if fired at longest range into FoW. Katyusha's 4 wave barrage does make it easier to retreat your infantry without getting the full brunt of it - but for slow or static positions that is a non-issue.