Case in point: Open source mailing list archives. For various reasons, some times I have to search for entire email threads that are available in the web interfaces of some mailing list archives. I have part of a phrase of some email, I put that into google with quotes, and it returns some results, usually with the main archive being in the first results. In the last year or so, Google has started returning no results for some phrases of emails that do exist and are available in one or more public archives.
When that started to happen, I tried Bing. And Bing returns results with links to archive. So does duckduckgo most times. But for Google, it's like if that particular email I'm searching at that moment didn't exist (for other emails in the same mailing list it works fine). It only happens occasionally, but it's getting worse. So I have started to rely on DDG and Bing more and more, because they always find what I want.
So Google is starting to fail at some of the most basic aspects of a search engine, it's not just the ordering of the search results - there are some public web pages that it won't see for whatever reason.