The value of products is based on the SKU, which is not what I would have said if I hadn't just looked. If you have attributes that adjust the SKU, then they wouldn't ever be counted except when "All products" was chosen. And that's a problem.

The condition "order has products" uses the nid, and it seems to work pretty well. I think I'll use that as the standard until there's a better way to get all the possible SKUs that a product can have.