Just found this thread, but never post in this forum. Sorry for the late addition.
I have a bike with an S3X hub, and it's not a gravel type frame. I have done some easy gravel with, and varied off road riding with it. Gearing as shown is approx 70, 55, and 44 gear inches with the S3X ratios as they are. Have had great experience with the hub. It is heavy, but with the variety of gear inches it doesn't impact my average in speed, it actually helps. I like it a lot, but it has it's drawbacks.
With some wider tires/different frame and proper set up, if fixed off road is your thing, I think the hub can handle it as long as you are not using the hub/back pedaling for modulating speed.