In the current year the La-7 has been credited with more tank kills than the dedicated 190 F. I think that says a lot about how uncommon the use of the PB-1 really is (especially if you take into account that many of that tank kills by the 190F are made with bombs):
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As you can see, only about 0.9% of all tanks killed by planes have been a victim of the 190F and only a part of them had been hit with the PB1. In all of my 8 years of AH, I have been hit by them not more like perhaps 3 or 4 times. (EDIT: Actually I have exactly 2 deaths in tanks to the 190F)
The difficulties of using it compared to tank busting guns or bombs makes it a potent, but rare specialist's weapon.
Is it safe to assume that the La7 "earns" those kills by flooding the tank with damage points via the triple 20mms and then they get the credit when the targeted tank receives a kill shot from a bomb, AP round, etc, and that the M18, M3, M8, wirbles, etc, are included in that stat? It is very rare that I see an La7 take up the pair of 100kg bombs for anti-gv work. Too difficult for most players.

Lusche- you of all people should be able to consistently use the 190F-8 and the Pb1 rockets for anti-tank work. If I can, you can. Many people can, but the easy mode of the A20 and IL-2 (and evidently the La7, too) makes for a hard sell of the 190F-8, I know. Just remember the firing order of the Pb1 rockets (2 left, 2 right, and so on), aim to the side of the tank opposite the wing being used (left rockets = right side aim), and wait 'til you see the whites of their eyes then let loose the dogs of war (under 400 yards)!
There are many players out there that could successfully utilize the Pb1 rockets if they'd take the time to learn them, it isn't "rocket science".