Some challenges:

- it's hard to draw a line between true and false in the presence of subjective information.

- it's hard to do a classification without false positives and false negatives.

- it's hard to defend yourself from accusations of bias and manipulation in the presence of false positives/false negatives.

- it's hard to scale a process involving human reviewers.

So, in short, what they did is to let the community regulate their own content. If people visit some stupid article about curing cancer by eating cheetos while twerking then that is what is going to trend. If people want to read about Brawndo, the thirst mutilator and how awesome it is on plants let them read it.