Can anyone shed some more light on why Magento recommends against this now? Other than the short description they have in their dev docs.
Continued use of this feature is known to cause performance degradation and other indexing issues.
Flat indexers can cause:
- Heavy SQL load and site performance issues.
- Long running and stuck crons.
https://docs.magento.com/m2/ce/user_guide/catalog/catalog-flat.html