Stainless is hard on brass fittings because the flare is so hard that they don't deform to match the brass and make a tight seal.
The regular steel is somewhat softer and the flare will actually squish and help form to the shape of the brass, creating good metal to metal contact and a seal.
Because stainless is harder the brass has to do most of the deforming to match the stainless and make a seal. Once a brass fitting has been fit to one stainless line it is often damaged enough that it has trouble sealing on a different line, the brass seat is damaged.
Because of this it is often harder to get a leak free seal when using stainless lines. To reduce this problem a high quality flaring kit and some practice and patience are most helpful.
I personally have had minimal trouble with getting stainless lines to seal, no more than I have had with regular steel lines, but I am careful and make nice double flares.
Grigg
