A factory on an island will have access to the sewers.
A factory has plenty of things to use to fortify it.
May contain machinery and equipment that will be useful to your survival. Such as a factory that produces solar panels, bullets, weapons or other things of that nature.
If the factory is surrounded it will be hard to use an escape route
Bandits will raid the factory.
Survivors may have already looted the factory.
Will not contain much space for people to live in. They are not Warehouses (large open spaces) they are factories originally used to manufacture a product so will be built with the main intent of that product.