OK it's small and the components are complex. But as an assembly it's a cpu, a battery, a taptic engine, a speaker, a plethysmograph in a case with waterproofing.
Only two of these components has moving parts. As long as it's clean room manufacturing, assembly should score about the same as a watch.
Perhaps.
But those complex parts have to manufactured in large quantities as well. It's not just the final assembly that's the issue, it's getting all the components (including the case and the glass) in proper quantities so that the final assembly can take place.