All the major worktops currently available in the UK (such as Resopal, Duropal, Prima, etc) are single side post-form laminated on a dense-ish particle board core. The underside generally has a layer of vapour barrier material (coated paper in effect) applied which has different moisture resistance properties to the laminate applied to the top/front edge, i.e. it is somewhat more permeable. I think that is partly why manufacturers bag their products in plastic and are fussy about storage conditions. Store a worktop in damp conditions and it will warp or bow. Store it racked upright without adequate support (like some merchants and DIY places do) and it will definitely warp given enough time. Once in place in a kitchen the underside of a worktop is sufficiently moisture resistant to be fit for purpose, however, it is noticeable that some major manufacturers of built-in dish washers and washing machines now supply additional vapour barriers to be applied to the underside of the worktop. If worktops are moisture resistant to the same extent both sides, why do that? Try sticking a couple of end offcuts from worktoips out in the winter rain, face and underside up for a month and you'll see what I mean about permeability.
The back edge of a worktop is similarly treated by the manufacturer with a sealing compound, and a good kitchen fitter will reseal this surface should he find it necessary scribe the rear of the worktop. He'll also do the same with a bonding agent in the mason's mitre joints.
All the upstands I've seen to date have a similar construction to worktops, and whilst weighting the item down may flatten it why should anyone have to accept sub-standard goods?
I have to admit that we get more problems of tension with raw particle board than with raw MDF - it results in a "banana" edge when strips are sawn on the panel saw. Any sheet showing those tendencies ends up getting machined on the CNC where we can clamp the so and so down so it won't move. Laminated materials such as MFC and MF-MDF tend to be more prone to this, especially the single-sided stuff used in retail stands and the like. But on the odd occasion we'll get a sheet which warps in both planes.
Scrit