It’s the old swings and roundabouts.
The disadvantage is the extra weight – rotating mass where you don’t want it.
The advantage is that their performance on almost all surfaces is such that you don’t notice it.
The big advantage is you can ride your bike in places where your other bikes can’t go. The big disadvantage of that is when you stop and put a foot down, you sink up to your knee 🙂
If your riding is aimed at taking you places, then they are unbeatable.
I’d say that for 25% of my riding they are much better, for 50% they are no disadvantage, and the remaining 25% is only a slight disadvantage.
It’s that 25% where they are better that enables you to get off the beaten track and really enjoy mountain biking as opposed to riding along maintained tracks.
I’m now beginning to wonder if there are only 2 sorts of bike worth having for mountain rides – a fatbike that can be ridden almost anywhere, or an ultra light SS CX bike that can be shouldered while you run/walk/clamber over the worst stuff.