I'm not sure, i know it will bleach them out and cause the colors to fade, but i don't think it will make them brittle.
Okay, from what i can find, not to any degree you'll probably have to worry about. ABS Takes a while(by which I mean a year or so), but it will slightly deform. I'm sure most materials will. If only because heat makes things expand, so with the cooling and heat of night and day, it will probably cause stress from the constant change. So I guess it comes down to the strength of the material. Obviously rubber bands aren't near as strong as plastic. TBH, I could also be compleely wrong about this. I'm just going off of what I can find and my basic High School knowledge of physics and thermodynamics.