Why? Windhund in B gets more numerous infantry in trucks and they get Jagdpanzer IVs (9th Panzer really needs JPIVs in B I feel since they are currently stuck with just Panzer IVs and Marders until C) which are pretty solid. Phase B income is pretty bad though.
I say they're dead in the water because your opponents will have every advantage in phase B:
-Any Allied deck that's not Commando will have more income than you.
-You start to get infantry at more than 2-per-card but with no vet, and you probably can't put too many cards into them because you don't have a ton of infantry slots and need to devote a good portion of them to phase A because you get so few per card in phase A. Consequently, you're probably still going to be inferior in infantry fighting to whoever you're fighting.
(the most common exception being if you end up facing off with the 3AD)
-Phase B is a weak spot for Windhund in terms of the equipment they have available to them. They're actively weak in the tank tab because you're reliant on the vulnerable and overpriced Pz4. Other tabs largely lack anything exceptional, with the exception of the JagdPanzer 4.
The JagdPz4 is a nice piece of hardware and should do a good job of locking down open fields because the Allies are going to have a hard time penetrating it. However, it's expensive, doesn't have a turret, only carries 10 HE shells, and only has one MG, which limits its utility for pushes.
The Windhunds are nice and well rounded in phase B, but in general it's going to be a weak phase for them. If your opponent knows what they're doing and isn't already in a disadvantageous position, you're likely to spend it struggling to hold the line.