It affects all classes.
Not in the same way. My biggest strength is speed and acceleration. Rain makes me slow just like everyone else.
STR oriented build has high PS therefore deal shitload of damage. Speed and acc aren't his best features. When it's raining his damage stays the same and he's just a little bit slower than AGI build.
Also I think that horses don't give a fuck about rain.
Edit: Rain and nighttime effects in this game sux. They add nothing to the atmosphere because they are so bad.
If you care so much about such details why you don't add that mod which would make flailing flags possible?
Playing c-rpg during the clear day feels good. Playing when it's raining makes me feel really depressed and playing during night makes me wanna kill myself.