If I had an unlimited budget I would build oe of those 24-pounders I just read about.
I used to ride some trails with deep sand sections where the only technique was plow forward several feet, fall, and flow forward some more. The fat bike would have been a blessing---and at five pounds lighter than my F/S Cannondale, it wouldn't have been a liability on the rest of the trials.
Right after I get a CF road bike, and a Ti road bike, and a CF F/S MTB .... and about $15 K in camera gear .... after I win the lottery, I guess ...
I rode some miles last summer with a former MTB racer who was really intrigued by fatbikes ... I wasn't, but I didn't know they could be built so light.