Why is this a problem? If all rockets are going to hit, the flak HT was placed on the front line and it should be an easy prey for arty. At anything other than point blank range, there is no way a katyasha is going to take out a flakHT in one attack. Just because a building has a cannon on top of it doesn't make it a front line tank.
but this also means as long as you have a katusha and you bought ~20 seconds for your katusha to do a 'fairly close ranged' barrage, it guarantees the destruction of a base building and time until the new truck comes, sometimes. i think that is too much.
but i also love katusha rockets doing actual damages to tanks when they penetrate enemy armors albeit rarely.
maybe katusha should do 50% damage to buildings? i use katushas a lot even before the recent patch and i thought they were serviceable. now, they kill ambient buildings and okw base structures like there's no tomorrow.
EDIT: can walking stuka not one shot building please?