Damn...
I wasn't really expecting a lot but I was hoping they'd at least be able to deal with a bit of rain.
My experience is a little different than others...
I have an actionpacker (the medium size) on my roof rack w/chains, tools, jumper cables, etc. In the groove on the lid I added 3/4 foam strip (used for weatherproofing around windows). It's been on the roof for 2 Pacific Northwest Winters, strapped down tight with a tie-strap. Snow, sleet, hail, sun, etc. Took it down a couple weeks ago to get out the jumper cables. No rain, no moisture, no rust, no issues. I've got a few others that I've waterproofed the same that don't spend as much time outside, but have been like that for 5+ years with now issues.
I would imagine if you opened/closed it regularly you might have weatherproofing issues, but honestly I wouldn't hesitate to leave it up there in just about any weather (it doesn't get much wetter than the PNW in winter lol).