Either they are or they aren't

While one site says,

Public defenders usually work in “niched” areas of legal specialty, such as DWI or domestic violence defense. So they tend to be up-to-date on new law and legal theories in their area of specialty.

another site says,

this lawyer, known as a public defender, will not necessarily have any expertise in the field in which he is defending the client. For this reason, it is to the benefit of the client to hire a criminal lawyer with expertise in defense, as it is an extremely specified field.

(Sure, the first site is sponsored by LexisNexis and Martindale-Hubbell, but the second site cites Psalm 146, you know, the one about the One who executes justice for the oppressed, gives food to the hungry, and frees the prisoners. That's worth something.)