1. Field mapping you can control
If you cannot override titles, product type and custom labels, you cannot do feed optimisation. That alone rules out several popular options.
2. Identifier handling
GTIN, brand, MPN and the identifier-exists flag need to be mappable per product. Missing identifiers are the most common cause of catalogue-wide disapprovals.
3. Multi-market behaviour
If you sell in several countries, the app has to send the right currency, price and availability per market. Getting this wrong is a misrepresentation finding, not just a data issue.
What does not matter
Dashboards, AI badges, and "optimisation scores". The feed is judged by Google, not by the app.