got minor rust on a vibration isolator doing a job in fog for a week.i Prob wouldn't run in rain; can't protect vulnerable parts as easy as Alta, movi, etc. will likely work but Coat metal parts in anti rust, avoid mud, and take apart and dry after. That's what I did in wet fog. Doubt freefly will recommend rain. I would avoid. Good luck!
Yes the Tero isn't exactly waterproof, the ESC is stated as such by the manufacturer and that is the most vulnerable component. Saying that, we have run ours in light rain for maybe 10-20mins and it was ok. We also have had it very muddy from a week on a golf course and it was still fine. As long as you clean it as soon as possible and keep it stored dry. I've completely disassembled ours to clean it a few times. As Sebastian said the Vibro-Isolators are susceptible to rust as they along with screws and springs are the (only?) non-aluminium/plastic parts.