I don't have a Radbot, but I've been running a Fenderbot in Seattle for about two years now, no problems with rain/moisture. I'm pretty sure they're the exact same light, the Fenderbot just being set up to bolt onto the back of a fender instead of having other mounting options.

I can also recommend the Mars 4.0, super light, bright, some side visibility and no problems with rain or moisture in two years of commuting in Seattle. I commute 3-7 days a week, about an hour each way, year round.