For the invisible Buildcraft pipes, midasGold only converts those in chests and in player inventory. All placed Buildcraft pipes are a single tile Entity, so they are not converted, and buildcraft doesn't recognize them unless they have the same pipeIds as 1.2.5. They can be edited by hand with NBT Explorer, but it is a much more time consuming process.
For condensers/alchemical chests, they could be converted to crystal chests, but when converted are empty. If they are converted by hand, they keep their inventories. Grrr.
So converting a Tekkit Classic world works for the most part, but it does have some challenges.