They don’t seal as well, I made a mistake purchasing stainless steel the first time. If the flares are off and/or the harder metal, it takes a while for them to seat, you gotta keep backing the nut off and on.
There is a post somewhere here that detailed these little copper ferrules that they use in HVAC. I guess they call them flare gaskets. Use those as crush washer and the SS will seal just fine. I got my preformed lines from Cass @ DoctorDiff