First result on google for civil forfeiture. Followed up by searching for [law enforcement agency] and union returns zero results for the worst locations.

Police officers are unionized pretty much everywhere. Sheriffs, which carry out the most egregious civil asset forfeitures (usually in small travel-through towns), generally are not unionized outside of the big counties and states.

Mine: Unions have a direct, negative influence on the ability for departments/cities to fire bad cops.

Yours: There is correlation, but not causation, between non-unionized police departments and instances of civil asset forfeiture.