There is always a possibility of police abuse. There is often abuse found when people are in positions of power, but mental health screening can help identify those with the potential to abuse others and can help reduce the amount of abuse that goes on in the police force as a whole.
I do think this is part of the reason why police brutality has become more prevalent--or, at least, seemingly more prevelant. We should be able to trust the stability of our police. Another reason is because everyone has a camera via their phone all the time now. And it gets caught more often.
There is much abuse and unfair treatment in the enforcement. What ever happened to equality? With the police avoiding mental health examinations, what if they are hiring a physco? Is that the type of person you want holding a gun? Perhaps much of the police abuse (and murders) may have been avoided if they have been screened for any illnesses.
Police abuse has not become more prevalant because some departments have done away with mental health examinations. Mental health exams can only provide a glimpse of how a person is psychologically at the moment, they do not account for the experiences that may change a persons psyche after being hired.
I am not sure that a mental health exam is going to be able to predict which police officers are going to abuse the people they are arresting. What does happen is that for some people the power goes to their head and they somehow think that they are allowed to act like a bully or a vigilante in the name of justice.