Thats the weakest point of Wstukas, their high risk/high reward is the surprise effect of long range accurate volleys.
Problem that you don't have any chances to retreat your team weapon on whole front when Wstuka fires. Any small moment that you missed and you lose very big amount of troops. No any rockarty could give you that. Katy could shredded blobs, werfer could too. But these are blobs, blobbers must suffer, especially when rocket arty on field.
And again - routes of retreat. If we could change routes to retreating squads in live action - Wstuka didn't be so much shock and then OKW players will be in hard situation, when enemy could evade even retreating squad your precise strike. It's just live exploit of situation where player don't have any power. You just unwilling viewer of doom of your soldiers. It's not fun.
If you can see where it is aiming is rather easy to detect the firing pattern.
Just remember old b4 laser-guided ability? It was fun when you heard sound of shot (and you have some thoughts about it, especially when repairing your panther) and know that it will hit to you? Very fun to play with in russian roulete when your opponent have 0 risk?
Wstuka the same exploit, couldn't kill p4, but could ruin all your defense in one volley and make huge MP drain.