Why does iTunes Store Reviews RSS feed sometimes return no results?
While this answer isn't very satisfying, it's the best I could work out after many trials. It appears as if the XML feed is really fallible and shouldn't be used for real-world usage. Furthermore, when using the public JSON feed, certain fields such as review date are missing. Neither feed reports developer response.
It's also clear that Apple doesn't use these feeds for iTunes (desktop) or App Store (iOS). I ultimately reverse-engineered the way iTunes requests review data and figured out that making a request the same way, making sure to match their User Agent and version, would return the data I needed. These requests seem to be rate-limited to a certain extent and the data comes as a mix of HTML and JSON that requires a lot of parsing. Furthermore, because they're private calls, Apple could easily shut the door at any moment.