There was a lot of drag on their axles as far as I know. They were a hard slog up gradients. with the wheels being that small a bit of dirt or grit can derail them easy enough too.
Having said that they're really nice wagons. Perhaps Dapol made improvements over the first batch. The wagons themselves need to be weathered mind you, the blue is really bright and the real ones got manky very quickly.
Believe it or not there has been two Eddie the engines. The first is the one you have (I have one too, sweet loco) and then the second was when DB got the Stobart gig and painted up 66 048 (I think that was the number) Anyway it derailed on it's first outing for Stobart in Scotland and has since been dumped in Toton looking very sorry for itself. A write off I'd imagine.
DRS also painted up a second loco in Stobart livery and called it 'James the Engine'; 66 412 I believe. Both since have emigrated to Eastern Europe!