I'm using current commerce 2.x.dev for online store development. It's first project with Commerce 2 for me.
When i started to work on products import, i found that Feeds module does not stable, and i decided to write custom solution for data import (Batch/Queue API data import from CSV/XML sources).
So, at this moment i cannot find any information about correct product entities creation via code. I explored Drupal Commerce documentation section: http://docs.drupalcommerce.org/v2/product/products.html but it contains only UI instructions for manual products management.
I think that short instruction for working from code with products / orders entities will be very helpful for developers, especially for developers, who starts working with commerce 2 and have some experience with 7.x commerce.