A Reddit thread is littered with people that tried performing the update, only to have their Apple Watch Series 4 become bricked during the process. In this case, "bricked" means that the Apple Watch is stuck on boot screen which displays the Apple logo. Attempting to reset the device (pressing and holding both the digital crown and side button) does nothing, and only brings back the dreaded Apple logo.
Apparently at this point, the only solution is to send the device in for repair or do an express replacement (either in store or via mail). Redditor kevan0317 expressed his frustrations with the situation:
Since I purchased AppleCare+ I would normally be able to just run to the store and get it replaced in-person but since everyone is out-of-stock they have to mail me a new watch and then I mail this bricked one back to them. My $600 watch is now completely useless two weeks after purchase and I won’t have another one until either Friday or Monday of next week.
Color me slightly frustrated... I bought the premium product so I wouldn't have to deal with this kind of issue. I guess no one is immune.
For its part, Apple has acknowledged the bricking issue, and provided the following statement to CNET:
Due to a small number of Apple Watch customers experiencing an issue while installing watchOS 5.1 today, we've pulled back the software update as a precaution. Any customers impacted should contact AppleCare, but no action is required if the update installed successfully. We are working on a fix for an upcoming software update.
As a result of all the problems, Apple has pulled the watchOS 5.1 update. For anyone else that has an Apple Watch Series 4, downloaded watchOS 5.1 to their iPhone, but haven't yet updated -- we suggest that you avoid doing so until Apple can provide a less destructive update.
Many Apple Watch Series 4 owners were eager to install watchOS 5.1, as the update enables the electrocardiogram (ECG) feature that allows you to diagnose heart conditions like arterial fibrillation (AFib).