Tara Lara Lou Lou
I think that this app is slightly misunderstood. It is not a solution but more of an aid. In other words you will need to expect error messages when you upload your catalogue to Google Merchants. This is not because of the app, but because Google requires certain information such as a category and barcodes or part numbers etc. which in my experience is not automatically added when you create your product in Shopify.
We have a large catalogue of hundreds of products so entering them in the Google spreadsheet manually would be too time consuming. This app has definitely reduced our workload.
As a suggestion going forward, I would ask that the developers find some way in which format the Shopify product page so that we can add the Google Merchants data at the time of creation and therefore save time.
Even though this is a free app, I think the developers should set expectations that there will be some tweaking needed.
If you have a large catalogue of products then I would definitely recommend that you use this to help you.