IMO derailleurs are not bad at all in the rain. When I have trouble is when they ice up. As far as rust, my problem with that isn't so much the rust as the clay/sand mixture they put down on the gravel roads around here. If I were on pavement all the time, and only riding when it's above freezing, I wouldn't have much use for an IGH. I don't actually have one but I think I'd really like to have one. I would already except for the need for a horizontal dropout or other chain adjusting hardware.

I'd probably give the NuVinci hub a try myself. The newer version seems pretty decent and not horribly heavy.