I've researched the claims of many "mainstream" news articles and found almost zero falsehoods which would stand up in a court of law (courts are very lax and consider many statements to be matters of opinion that I would consider to be matters of fact); however, I've found pretty much every article, most sentences have excessive manipulative framing that taints or spins a story, leading the reader towards beliefs which are very often untrue, unproven, or judgemental (normative). I noticed this as a teenager in the 1980s and deliberately tried to 'unspin' stories, but gave up after realizing how much work that was going to be. This isn't something that happened in the last four years... at least not for right-leaning people. Maybe it's a new idea to some left-leaning people (other than Noam Chomsky/Edward Herman).
As for "non-mainstream" most of them are much worse, often containing actual provable falsehoods. But some of them do seem to be more accurate and less manipulative. I have to say "seem" because the ones I respect the most have the same biases that I do, so I can't see the mistakes as clearly.