montalban, AEGs are fine, as long it's not -35C. Battery might not last long, but range and power stays the same. GBBs usually don't work well in cold weather. I haven't noticed any problem when playing in the rain. Maybe if you have open wiring in the mechbox or battery water will cause damage but if it's all secure nothing really happens. I try not to let water into the barrel to avoid getting internals flooded.
When it gets really cold grease will freeze and plastic/mechbox shell will most likely crack. AEGs also have different mechbox versions. Version 3 is most durable.