Under slung can be a PITA offroad, especially if the going is soft, ergo muddy as fcuk!
Because I like fat tyres, the underslung 305 spare used to bottom out too, as well as being a tight fit getting a 33" in under there. It's a mucky place to keep a spare too, the rims degrade whether using steels or alloys, it's just not a good idea IMO.