Like I said, Snopes is great for debunking certain items & has a rock solid reputation as such, but even if someone sent me Snopes tiered analysis of all of Harry Reid's shady business dealings I would be suspect of it. Not because I don't think Reid is a despicable, shady mofo - it's that Snopes doesn't have that sort of a reputation for going deep on such issues.
A fact checking site can be 100% accurate but if it doesn't apply the fact checking evenly, that's bias. Politifact is a site that gets referenced quite a bit but there's no doubt it leans left. If I operated a site that dealt with the biggest lies of Hillary & highlighted her worst ones & then fact checked stuff like, "Is Trump's hair real", or if I did write about a lie of Trump but then spent paragraph upon paragraph trying to offer an explanation far greater than anything Trump has, it would be disingenuous not to admit to the bias.