In the context of criminal law, hunches are not enough because more concrete proof is needed in order to do things like securing a warrant or gaining a conviction.
When police are investigating a crime, it is not sufficient for them to have hunches. They cannot get a warrant because they have a hunch they would find evidence of illegal activity in a given house. Instead, they need to have probable cause that can be examined objectively by a judge.
When a case is brought to trial, it is not enough to have a hunch about the defendant's guilt. The prosecution must have enough evidence to prove the case beyond a reasonable doubt.
Thus, in the context of criminal law, this phrase means that objective evidence is needed and that hunches are not sufficient.