I would say that if you leave a Dolphin controller out in a rainstorm then you’re asking for trouble. It may not go bad right away but you’re definitely going to shorten the lifespan or open yourself up to failure. The controllers are not repairable, only replaceable. So a failed controller outside a warranty replacement is going to cost you $200-$300. I always suggest getting a caddy as it makes wheeling the unit around and storing it MUCH easier.
If you absolutely can’t or do not want to take the controller indoors, then I suggest you put a small plastic equipment shed or deck box (like a SunCast or similar) near the equipment so the robot can be stored under cover. If they are not on a caddy, then the Dolphin needs to be stored on its side or upside down to avoid smashing the brushes. Some people cut short lengths of 2x4 and store the unit on those so the brushes remain suspended.