Not at all! Private investigators are the ideal way to gather more evidence. The police only have so many resources to work criminal cases. A private investigator is your best bet to uncovering the truth in civil matters. Certain types of work such as infidelity cases fall outside of the police's domain. In these instances, you either perform the surveillance yourself or alternatively, you hire a private investigator.
A private investigator is ideal for several reasons. For one, their entire job is to work cases such as these. As a result, they have much more time to dedicate than the average person. Additionally, they work with much less regulations than the police do. This allows them to get creative in how they do their job.