Expungement rules and effects vary greatly by state. Good reading on the question can be had here, with notable exceptions to expungement here. Of particular relevance to this question:
In some states, individuals who want to work as public school
teachers, corrections guards, or police officers should expect that
their employers will have access to expunged records. Agencies
reviewing applications for professional licenses, including law,
pharmacy, or medicine, may also have access.
Even in the most favorable circumstances an expungement can't destroy or seal non-government records. For example, if a newspaper reported on a charge or conviction there is no way to eliminate that public record.
The mechanics of expungement can also break down. For example, in Pennsylvania it's up to the applicant for expungement to list all the government agencies and entities on which the Order for Expungement should be served. If they forget or aren't aware of some agency that has records covered by the expungement then those records won't be destroyed. (Though if they are later discovered the Order can be served on them and they are still required to comply.)