By far the two best companies are Bose and Logitech.
Bose has twice sent me a new L1 overnight and paid shipping both ways. Once I called them about noon, explained the problem and I had a new unit on my doorstep the next morning! They also sent me a box of fuses for the first L1, at no charge.
Logitech will ask you a couple of questions on the phone and if it's apparent that there is a problem they can't troubleshoot, they just send you a new unit and tell you to keep the old one!
As to keyboards, I've had good experience with Yamaha, no experience with Korg. Bad experience with Roland, but that was years ago.
The new Ketron distributor has been excellent in replacing faulty parts and keyboards. Of course they were new units and seem to have been damaged in shipment.
It has taken a long time when the part must come from Italy though. Those people are on their own schedule.
DonM