In spite of Shimano's ass covering, there are lots of people using Alfines for mountain biking. I've used an Alfine 8 for probably the last 4 winters, - the last two of which I'll acknowledge were pretty wimpy.

But I haven't had much trouble with it. Which cassette joint were you using? The wet weather one? That's what I have and haven't had much trouble with it freezing but cable corrosion caused some problems.

After the first year I did replace the grease and did an oil bath instead which has improved cold weather performance. The Alfine has not been as low maintenance as I would have hoped but it's been better than any derailleur I've used during the winter. Towing a trailer is not something I've tried in cold weather though. At least not on a regular basis.

Another alternative to a derailleur would be a different IGH and if you were to go back to derailleurs, I'd go 1 X something. The front derailleur is what always seemed most susceptible to ice and grit. I remember trying to kick them loose with my foot.