This is a question which has interested me for some time. The short answer is that if they are kept reasonably dry - they can handle the occasional splashing and certainly UV - Sunlight., certainly dry cold, certainly winds.

The enemy seem to be constant wetting, moss and algae. The lime effect of killing algae only lasts for a short time.

This observation is from Vancouver Canada - where we get about 6 feet of rain a year and sudden frosts.

An outdoor fresco thrives under the eaves of a house - changes with the colors throughout the day - and for a kick - in the summer you can spray the hose on it and it comes alive with the bright, fresh color the day it was painted.

Another problem is that a rough surface can attract dirt/soiling. The only way to properly clean it is with one of those 'dry' steam cleaners. So some people prefer to finish fresco with a very polished surface.

Keep in mind that even with aging - this is a natural, architectural patina - it is hard describe the beauty of this aging in place. If you plunked this object into another place - it would not quite fit. It would an object - not a part of the scene. You are an artist - you will know what I mean. This is why some where depressed by the cleaning of the Sistine frescoes for example. These are the sort of topics which we rage over bottle(s) of wine and fine bread I guess.