For guarding the most evil and demented of dark wizards, use of Dementors as prison guards almost seems justified. The strain placed on these evil wizards is hardly a concern, as many of them took many lives and ruined families during Voldemort's first reign of power.

There are two problems with using Dementors that I can see. First, Dementors can be viewed as a cruel punishment when innocents or individuals who committed minor crimes are involved. Just as the muggle justice systems of the world occasionally convict innocent people, wizarding law enforcement is no better. To those that are wrongly convicted, subjection to Dementors and potential madness is an inhumane, wrong, and irreversible punishment.

Second, it is questionable as to how much wizards and witches are capable of controlling Dementors. As evidenced by the Second Wizarding War, Dementors were able to rebel against their wizard "handlers," demonstrating that understanding of their biology and habits was shaky at best. It is certainly not wise to trust our most hardened criminals in the hands of dark creatures that we neither comprehend nor are able to control fully.