I bought my 17" MBP as a refurb and it worked perfectly for several years. You can expect Apple refurb products to be just as good if not better than new ones. I'm saying better because Apple applies more stringent QC on its refurbished inventory before offering them up for sale again. If there is a tiny flaw - say a micro scratch on the case - Apple will swap the defective part for a brand new part.
Every official Apple refurbished product I've purchased has been indistinguishable from brand new, only the box has been different. Although I've not bought an Apple refurb watch, I would expect the same.
... and if it should happen that it isn't to your liking, you have the same 14 day return policy.
My very first watch (series 3) was a refurb from Apple, there was no way to tell by looking at the watch that it was a refurb; it looked (and worked) as though brand new. As others stated, only the box was different. This has been my experience with 2 MacBook Pros and 2 iPads as well.