In my experience the biggest problem with fact checkers is that facts are complicated.
Somebody says "the speed of light is 300,000 km/s" and you go to a fact checker and you get:
> "TRUE: The speed of light is approximately 300,000 km/s."
Then you go to another one and you get:
> "FALSE: the speed of light is actually 299,792,458 m/s in a vacuum and has a medium-dependent value through air, water or another medium ... "
Then you get into an argument with someone where the distinction matters and they cite the first one, or get into one where it doesn't and they cite the second one.
Somebody says "the speed of light is 300,000 km/s" and you go to a fact checker and you get:
> "TRUE: The speed of light is approximately 300,000 km/s."
Then you go to another one and you get:
> "FALSE: the speed of light is actually 299,792,458 m/s in a vacuum and has a medium-dependent value through air, water or another medium ... "
Then you get into an argument with someone where the distinction matters and they cite the first one, or get into one where it doesn't and they cite the second one.