I can't imagine it's less reliable than, say, the field of sociology. At any rate, the claims in the wikipedia article are well-sourced, so if they are false, it should be trivial to demonstrate as much.
> it's not evidence. That's just one incident in among claims by millions of people that have been going on for generations.
What you describe is evidence. It isn't proof. But your broader point stands--we need more data about this to know how prevalent it is. My only point is that it's not unprecedented for communities to lie--specifically your refutation ("it's been going on for generations") supports my pet theory, which is that a lot of historical oppression by police has turned many black communities against police, driving uncharitable interpretations of police behavior at best and outright slander (as in the Ferguson case) at worst. Note that these communities can be lying and not necessarily be bad people--slander is morally wrong, but it's also an understandable (though unjust, misguided, etc) response to oppression. Compassion and honesty aren't mutually exclusive, as we are often tempted to believe.
What is this statement based on? Can you provide any data? What about people - whole communities in many places, over generations - who claim to be persecuted continuously by police. Are those people making it up? The people protesting in St Louis - what are they spending all that time and energy on if there is no problem?
This issue has been discussed heavily; I think more 'IMHOs' don't add to what we know about it.