Really? Ever seen one not operating flawlessly because of wet conditions? There's even a video on youtube showing someone firing one under water. I wouldn't recommend using it under water, but I can't see a situation where a half decent AEG would not do the job.
I think TWG suggests not to use AEGs in sub freezing temperatures, as it might render the gearbox shell fragile. But I have never heard of this happening and we play a lot in those conditions here.
As for Tippman, they are hardly the only decent gas system, and I could probably find an environment where it does not work.
Oh and never leave your batteries or gas exposed to the sun and make sure your battery compartment does not get too hot.
tl;dr: Any decent AEG should stand almost any weather. But you might want to use one you can afford to replace or repair (probably a sound advise no matter what conditions you play in)