When the Soviets were converting PzIIIs to SU-76Is, two weak links were discovered in the "donor" tank, namely the engine and tires.

However, these issues are not even close to what the Tiger and Panther suffered from. Survey of Allied Tank Casualties in World War II also covers German tanks left over on the battlefield. Out of 100 PzIIIs, PzIVs, and StuGs, 40 were abandoned due to breakdown, and 60 were destroyed in combat. The Tigers and Panthers were not so lucky: Guderian estimates that 60-70% of Panthers were lost as a result of breakdown rather than enemy action, and a POW (tanker with 8 years of experience) is quoted as saying that over half of all Panthers broke down during march rather than during fighting.