That last comment would fit my experience. I've said it before but Holland IS flat and there's not much climbing standing on the pedals here. The Dekerf softail is the master of covering twisty, rutted and rooty singletrack passages at speed. I've not noticed any drawbacks on hardpack sections connecting the twisty bits, but logic dictates it helps if you spin it smoothly.
I don't recognise the thunking comments much, though I am a bit of a Clysdesdale? I guess if you tackle more demanding terrain you will hit the limits of the simple suspension designs.
Enjoy!!