It's extremely difficult because the human psyche seems incapable of distinguishing "fact" from "that which supports my view", at least when the emotions are activated, and the only topics where people seek "fact checking" in the first place are the ones where emotions are activated. There may be exceptions—i.e. people whose minds don't work this way—but if they exist, they're so rare as to be no basis for social policy. (And I doubt that there are really exceptions.)

That's why there doesn't seem to be any fact checker whose calls aren't predictable from one of the major ideological partitions.

Another way of putting this is that the question, "what are the facts?" is complex enough to already recreate the entire political and ideological contest. It's understandable that people would like to reduce that contest to a simpler subset of factual questions—but you can't. Just the opposite: that apparently simpler subset reduces to it.

(Edit: it might be worth repeating that there are infinitely many facts, and they don't select themselves—people do that, and when they're people with political preferences, that's usually the high order bit. https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...)