I use Stars and Flags to classify images. I also use Favorite. But I want to use those classifications in Photos.app smart folders. It seems that only Favorite is accessible from Photos.app. Which means that stars and flags are not really useful for smart folders. Is that correct?
Smart Albums in Photos can be limited to specific albums. Since Ratings and Flags are represented as Albums in Photos, you can use the Rating / Flag Album in the query though it’s not as good as what you can do in Nitro.