At minimum, there is no authentication endpoint for HN users, which is the primary reason you haven't seen many HN apps take off in the past 2 years.

A more damning reason is that the official HN API in its current state is worse than the API it replaced! The Algolia API (https://hn.algolia.com/api) is still active, and can retrieve data with 1000 entries per page (vs. 1 at a time for the official API), and can also retrieve the comments plus text of a submission thread in a single HTTP request (the official API requires the user to perform a HTTP request to retrieve the text for each comment in a thread)