Different things work for different people. The following should be of interest (this is a clickable link)
As far as the Apple Watch, series 1 and two don’t have location services.
The brand new Apple Watch Series 3 does have location services, but it doesn’t work the same way that a phone does. In order to help preserve battery life, instead of regularly broadcasting its location, the location is recorded internally and is available for query by approved apps.
That means that you can use “find my phone” to find an Apple Watch Series 3, but you can’t use Geopresence like IFTTT location.
So I’m guessing it won’t work with SmartThings, because you won’t get that “cross the border” Geopresence event. But I haven’t tried it to be sure, I don’t have a series 3 watch myself.
Well less than 1 day into trying to use my iPhone instead the results are pretty poor. I went out to dinner last night, and 5min after getting back home it informed me that I had just left. It’s yet to tell me I have arrived again!
I wonder if something could be made that checks for the presence of a device on the wifi network, and can be aware of presence that way…
There is detailed discussion of A couple of different Wi-Fi presence methods in the presence FAQ so you can see how people have already done this with SmartThings. It will depend on what router you have, though.