It might be biased on what it chooses to fact check, but that would require more work than I care to put in to accurately verify it.
As far as the rest goes, how can it be biased it gives the explanation and the facts and you are free to read it and make your own decision. Nit picking about mostly true or mostly untrue or whatever subjective value they choose to give it is silly, read the facts and decide for yourself whether or not if something was a deliberate lie, or they just missed some details. If you are already at the site checking something you care about is it too much to ask to read the provided information, use their result as a guideline but make your own decision. Unless something is 100% true or 100% false, how you draw the line in between is always going to be subjective.