I was beyond impressed with Nintendo on this issue.
I had the desync issue with my left joycon, like many other people, and after exchanging it at the retailer once, and the problem persisting, I contacted Nintendo. He set up a repair ticket, sent me the info to ship it to them, and I did.
I had it back in about a week, good as new, with the problem resolved. I can't get it to desync now, even behind my back, with the switch 10-12 feet away.
Of course in the meantime, I had purchased a pro controller, so I'm not really able to take advantage of the now working joycon, but it's nice to know its fixed anyhow.
My only issue with this process is taking care of the shipping myself. I had to find a suitable sized box, come up with some padding to stop it from sliding around, and take it to the courier myself, on the other side of the city. I would have preferred them to send me a return box with label, where I could just pop the part in, call the courier, and have them come get it.