I think there is some confusion here (at least I’m confused). We have to separate out XMP (file system) from Photo library (database).
When using the file system, Nitro does read ratings from original images. However, I see that there is a bug where if you then do something to generate a Nitro XMP file, the rating is discarded. So, I will fix that.
When using the Photo library, Nitro does not read ratings from original files. That is mostly because the Photo Library does not provide them to me. The only way for me to read them would be to read the original file. However, with the Photo Library, that could be in iCloud. So, I would have to download every image, just to see if there is a rating in the original file. I don’t want to do that. If people wanted, I could add a feature where you can select images and it will download them and read the metadata. I just don’t want to download every image “just in case” there is a rating, since it’s uncommon.