In general, the D4BF is just not the quality of the original 4D56T, Hyundai and Mitsubishi definitely make different quality products. That said, a brand new Hyundai Diesel is going to be more reliable than 30 year old Mitsubishi engine. We've installed around 30+ D4BFs and all of our customers have been happy with the swap. We did have a few engines that were missing a gasket which caused leaks, Hyundai did in those cases warranty those and issue a corrective action.
That said, we've definitely learned a lot of places do not know how to perform the swap correctly, and we've had to fix a fair number of mistakes from other shops.