The left Joy-Con desynching from the Nintendo Switch was a news item leading up to the release of the system and just after the release of the system. It turns out that Nintendo has identified the problem and they say that they've fixed the Joy-Con sync issue.
Gamespot is reporting that Nintendo has said that the desynchronization came about from a manufacturing mishap for select Nintendo Switch units during the production process, and that they've identified and rectified the calibration issue at the plant. This would explain why only select units were having the issue and why the issue was only present for certain Switch units.
Originally, some game journalists had noted that they had desynchronization issues with the left Joy-Con. It was mentioned that moving the controller too far away from the Switch or too far to one side or the other would force it to desync. There were various videos posted up attempting to troubleshoot the issue, and some people stated that it was due to the Joy-Con losing a signal connection with the base Switch unit when it was obscured or blocked.
Of course, losing signal connection due to blockage didn't explain why some people were reporting that the left Joy-Con was desynching while they were playing games like The Legend of Zelda: Breath of the Wild and nothing was passing through the wireless range of the Joy-Con.
Well, it turns out that it was how the controller was configured during the manufacturing process. A select number of Joy-Con units were affected due to this misconfiguration at the plant, but Nintendo didn't detail whether or not this affected a large number of units from the plant or only a small batch. The company has been especially coy about the failure rate numbers for the Nintendo Switch.
For the most part they've been working extremely hard to shape the message for the gaming device and ensure that people thoroughly understand how it can be used and what it does. The desynch issue wasn't the only problem plaguing the Nintendo Switch. There were also complaints about the docking station scratching the device when people were docking it and undocking it, as well as some people having trouble getting the clips on and off the device.
However, based on the lack of reports from verified purchasers of the Nintendo Switch, it doesn't appear as if that many people ran into the desynchronization issue with the Joy-Con devices. Nintendo did, however, offer to repair the Joy-Cons for those who did have an issue. So, if you picked up a unit and you have the warranty intact, you can have Nintendo take a look at it if you are experiencing the desync issue.
According to GameSpot, this issue should be wholly resolved and you won't have to worry about it from here on out. This means that all of the retail batches that arrive on shelves during the next distribution shipment should be free of the desync issue with the Joy-Cons.
Now Nintendo can get to work on pumping out more games for the Switch, which are still fairly sparse at the moment.