This App only works will single variant products. It is not an adequate solution if you want Google Shopping to show availability by color and size. When I asked support about this limitation they gave an excuse about URLs and photos. I did not have any trouble adding all variants by calling the API directly to load my products.
Given that the App does correctly send one variant per product, and that probably meets some store owners requirements I give it two stars, but over all I was very disappointed. It took a lot of time to do this on my own, and most of it was duplication of Shopify's effort. They failed to take it to the finish line.