Both the Walking Stuka and the Katyusha (in its current form) have large issues.
The Walking stuka: Very long cooldown. Precise and easily defined strike area in a long line. "walking" barrage.
Thoughts: The Stukas characteristics are both an asset and a hindrance. The narrow line strike area allows for precision bombing, but also gives clear indication as to direction and danger zone to the enemy. Unless many enemy units are set up perfectly in a line there is little chance of hitting very many units in a general area and the creeping effect allows for a relatively easy side stepping of the strike zone for the alert player with good reflexes. The Stuka rarely wipes entire squads unless these are garrisoned or support weapons that take equipment damage and is destroyed (most often if damaged before and not repaired). It does wipe squads that are squadAI(tm) "bunched up" in a small poinpoint if a rocket lands right, there is scatter within the zone so this is by no means guaranteed. The main issue with the Stuka is its ability to wipe hurt and retreating squads by predicting retreat paths. This should, in my opinion, not be possible. Otherwise the "pros and cons" nature of the weapon and its long cooldown is a fairly good mix.
The Katyusha: Relatively short cooldown, large and undefined area with a lot of scatter, but very high AoE and strike damage across that zone. Fires "barrages" of rockets one after the other.
Thoughts: This is pretty much the same as the Stuka in the "asset and hindrance" aspect. The large and undefined area makes precision bombing impossible, but also makes predicting the actual hits next to impossible, necessitating a complete withdrawal from a large area of the map. Also wipes squads that are already damaged or squadAI(tm) bunched. Due to random scatter of several barrages, the Katyusha has a better chance of hurting a squad in one volley and wiping it in the consecutive volleys. The Katyusha is more problematic for the OKW than for the Ostheer, in that the okw keep bases on field and use "hard points" to fight. The OKW trucks take a large amount of damage from the Katyusha, and cannot avoid the barrage. In general I think this different set of strengths and weaknesses balances out well in comparison to the Stuka. However, cooldown needs to be significantly longer (comparable to the stuka) and the damage against OKW trucks modified. A slight (and I do mean slight) decrease of the AoE damage to allow some side stepping would probably be a good idea.
Blanket statements like "if the stuka is fine so is the katyusha" is hardly constructive. Both units have issues and different uses. Avoiding hyperbole, playing with and against the unit and learning to mitigate it rather than simply calling "op" and dropping the whole "no you <insert faction of your choice fanboy here>" would lead to much better balance discussions.
Exactly my opinion.