Beagle,
The DC-6 is widely regarded as the best of the propliners becuase of its relative realiability and very good economics. The straight DC-7 was also said to be a good aeroplane but was beginning to suffer from the complexity of its engines: the power recovery turbine idea seemed a bit like trying to get something for nothing. The DC-7C was a towering acheivement but really pushed the engines to the limit. I've heard it said by someone who flew all marks of Seven that the C was just a bit too big for the available power.
I've also heard that the engines were routinely run at high cruise power to meet the expectations of a travelling public with its eyes on the jets. Finally, with the high octane fuel these engines were designed to run on becoming unavailable, the really big turbo-compounds can no longer be used as intended, rendering the final propliners unworkable. Carlos Gomez would disagree with me (and more power to him - I hope his 7B is a great success for him).