I run an Alfine on my Pugsley, precisely because of issues like this. And because at the end of winter, I was typically replacing the rusted derailer. My Pugsley sees on and off road miles, and they salt the heck out of the roads. It was eating every exposed moving part.
But the Alfine absolutely helps with the slush-cake problem. I think the Pugsley is now the only fat-bike that has a 135 OLD in back to run a hub like that.
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Good night...and good luck