You hold the key
Your own lock on the unit. No shared access, no front desk, no gate hours.
Weather-resistant
Sealed against wind-driven dust and rain, which matters more here than rainfall totals suggest.
No access hours
On-site storage is available whenever you are, which a facility with opening hours is not.
What secure actually means
The lock is the weak point
A unit is only as secure as the lock on it. A shrouded or disc-style padlock resists bolt cutters far better than a standard shackle, which is the most common point of failure.
Placement is part of security
A unit positioned so its doors face a wall or fence cannot be opened widely even if the lock is defeated. Where you put it does more than most hardware.
Confirm the insurance position
Contents in a portable unit may or may not fall under an existing homeowner or contents policy. It is worth a call to your insurer before you load anything of value.
Keeping contents in good condition
Dust is the real problem here
In this climate fine dust penetrates further than water does. Sealed bins rather than boxes, and pallets or boards under anything sitting directly on the floor.
Leave air around the load
Packing wall to wall traps humidity against the metal. A gap around the perimeter allows air movement and reduces condensation on the inside surfaces.
Think about what heat does
Anything with adhesive, wax or a battery is at risk through the hottest months. Those items belong indoors, not in a sealed metal box in a driveway.