I've been subscribed to news.ycombinator.com/rss for a few years, but (more often lately) I have noticed that it does not update frequently, and many stories that have appeared on the desktop site (not "new", but on the front page) do not appear in the feed.
I have also been using 'news.ycombinator.com/rss' for years, I have no problem with it, but then I only fetch and view the actual content of the xml in real time, and don't have a reader storing and sorting the entries.
What I can say is, the /rss feed always 100% matches the frontpage at the time of it's fetch.
EDIT: looking at the /rss feed, it's missing a <guid> tag for articles, which maybe confusing some readers that rely on that tag for article uniqueness.
I saw that too. I used to run my own HN RSS feed that was just a pass-thru to HN's RSS feed, but with the comments as the primary link. No matter how often my feed reader fetched, I routinely saw links show up on HN Daily or HN Blogs that my pass-thru feed hadn't captured.
Eventually I learned that hnrss has a flag to make the comments the primary link. I switched to that, and no more problem.
I will give https://hnrss.org/frontpage a try.