Actually, it’s both.
People who live in wet climates know the effect that water can have on your antenna or satellite dish. Particularly heavy storms can disrupt satellite service, water can seep into connections and short them out, and if things freeze, the expansion that takes place as water becomes ice can absolutely ruin electronics, creating cracks that destroy shielding and shred plastic.
On the other hand, people in sunnier areas don’t have it easy either. Strong sunlight can create heat waves that distort satellite and antenna signals, and direct sunlight can oxidize plastic leaving it brittle and cracked. In the rare case when a real rainstorm comes in, all that oxidized plastic just tears away and that… ends up being a disaster.
So really it’s not an either-or situation, when you look at it. The real culprit when it comes to your outdoor stuff is the cycle of sun and rain. I know that’s a bit of a copout so I’ll say, that probably rain is worse. Water is worse. All of the real damage to electronics comes from water, at least most of it does. The rest probably comes from stresses due to wind. The sunshine can be a contributing factor as it causes plastics to expand and soften and eventually crumble away, but for the most part it’s water that does the actual harm.
Is there anything you can do? Water damage due to electric shorts can be mitigated by making sure you have proper drip loops on your cables and tightening your connectors can make sure water has no place to go. Sun damage is a little harder to deal with, because you really can’t put antennas or satellite equipment in the shade. Pretty much the only thing you can do is inspect everything carefully to make sure that you catch sun damage before it gets to be a problem, and upgrade and replace equipment as it’s needed.