In 2011, I covered the Pietrowsky lines and concluded that 5...Nc6 6.Bb5 Bd7 was not simple to deal with. White has to be carefull in order not to overextend.
The main line is 5...Nc6 6.Bb5 Bd7 7.0-0 e6 8.d5 (diagram)
Forces the exchnage of the light coloured bishop, doesnt it ?
9...Bxb5 10.Nxb5 Qxd1 11.Rxd1 (diagram)
Mission Exchanging is completed ! But does black have an advanatge ? Absolutely not, since black still needs a move to recapture the pawn on e6, but this is not possible since c7 is under attack.
So we can safely conclude that the Pietrowsky line does not offer any advantage for the defender, proviuded white plays is quitely - a task not easy for the Blackmar Diemer gambiteer.