The Soviet conducted penetration tests with the BS-3; 100 mm field gun couldn't penetrate Ferdinand's frontal armour at the range of 500 meters - that equals pretty robust . Again, the BS-3 could penetrate Tiger's frontal armor at distance up to 2 kilometres at any angle.
The biggest problem was the exhaustive combat use: these vehicles saw 3-4 weeks of continuous combat, with no maintenance between. Sure, the engine fires happened now and then, but it was the deadliest tank destroyer of the WWII and well liked by the crews.