For large vehicles like delivery trucks.. the roof access is difficult, limiting their use as vehicles for moving survivors. They don’t jackknife, but they can roll over easily. They’re more maneuverable and fuel-efficient than 18-wheelers, but they can’t haul as many supplies. Bottom line: delivery trucks don’t really fit into any niche. 18-wheelers beat them for moving very large amounts of stuff over the open road, while pickup trucks beat them in areas where maneuverability and fuel economy are important. Raid these for supplies and scrap, then use them to make road blocks.
They do have advantages, they share common parts that many other large vehicles have and they can transport lots of people.