Is bad kerning a general rule when it comes to free fonts? I noticed some of that at the first glance on Linux distros UI.

Well, there was a time when kerning wasn't even an option, so good kerning will have had to be grafted on to an old font.  Additionally, Microsoft Word used to train its users not to expect kerning, and obviously it isn't available in a monotype face, which nowadays can be used to signify 'computer'.  You may also find that repertoire is a higher priority than kerning - for an interface font  it's more important to handle Vietnamese than to be beautiful.  One then gets hit with the issue of reflow - if a font is used for documents, users will not be pleased when the page breaks move from one position to another, so improving kerning may annoy more than it pleases.  (Many people don't design their documents to be indifferent to where the line-breaks occur.)

There are fonts around where even hinting is missing - it's a massive effort to be paid for in pride.

Thank you very much for your summary.