I don't think this is true. With the advent of body cameras, cellphones with cameras and FOIA requests you can build a good case if someone violates your rights. The bigger issues are that there is little to no consequence of a flagrant violation of rights because the police union is VERY protective over its members.
The ability to make a case is directly related to how much free time you have and how much money you are able to spend.
We cannot just wave this away when the vast majority of people cannot take off time from work or afford to hire attorneys when their rights are violated.