I don't think so. 

uniqueIdentifier

is determined by the app from which you get uniqueidentifier and device

Device id won't change after a reset.

The uuid of an app won't change as well.

So I don't think the combination will change.

I think it should stay the same after a factory reset.  It shouldn't be that common a thing to happen, but if you you have a doubt, just try it on your own watch.

I have observed that it does indeed stay the same across a factory reset.  I would love to say I intended to test it, but it was more a case that I observed the behavior when I reset a watch to set it up for a different tester.  I already had the device id saved in a database, then noticed the device was now associated with a different user.

So unless you change the app uuid, the device unique ID will be the same across app installs and device resets.

LOL. I encountered the same issue as well. My solution is to store two fields. One is unique key from identifier + timstamp and another is user's identifier, which can be duplicate. In this case, I can execute multiple times without uk conflicts