Containers are built to withstand a lot, so your stuff will be safe from damage from outside. They are designed to keep things dry at sea, so they are completely weatherproof. Unlike large self storage warehouses, fire is unable to spread from one unit to the next. They are virtually vandal-proof too.
Some self-storage providers also sell or provide a high security padlock and keys. Usually, you keep the keys so you are in complete control of who has access to your unit.
The location of the self-storage compound and the operator’s security record also play their part in keeping your stuff safe. There’s more about how containers keep your stuff safe here…