You just have to read the websites, like https://12bytes.org/articles/exposed/npr-national-public-rad... or https://www.mrc.org/bozells-column/npr-admits-liberal-bias

The criticism looks like most criticism of news media: braindead reporters, selective coverage, etc., pointing to some bias, but not really enough overall to be conclusive. And associated with the same sort of people that would sign a petition to ban dihydrogen monoxide.

IDK, my experience is that news reports are OK for getting introduced to a topic but all the detailed/useful information is in blogs and databases these days.