We installed a positive pressure DVS-branded system at our old house (this was well over 10 years ago). This was a 1900s-build, 1970s semi "modernised", wooden cottage. Timber piles into clay. No insulation and generally rubbish joinery (i.e. typical first home). We had serious dampness issues when we first moved in to the extent where mould was growing on leather jackets in one of the wardrobes. 

After installation, we had 6 weeks of no obvious change and me thinking we had bought a lemon. One morning (i.e. literally overnight) there was no condensation on the windows any more and the house felt noticeably drier. House also became easier to warm. 

As I understand it, this works something along the lines of the following:

The DVS we had was about as simple as it got - fan would suck ("warm, dry") attic air through a filter sock and push it into the rooms. Very basically, if you mix 1000 L of 50% humidity air with 1000L of 0% humidity air you end up with 2000L of 25% humidity air. Mix another 1000L of 0% humidity air and you get 12.5% humidity etc. Our rooms were far too leaky to hold the overpressure of the additional air, and so inevitably it escapes under and around doors, window cracks and through floorboards. Obviously it doesn't push 1000L all in at once, it is a very gradual process. You're not pushing out the moisture per se, rather mixing in additional dryer air constantly thereby reducing the moisture content of what's left (homeopathic home ventilation?).

Eventually enough drier air was pushed through to take care of the additional moisture we had retained by curtains, wall linings and likely even the floor boards. The DVS didn't push moisture into these, rather the drier air shifted the equilibrium enough for the additional retained moisture to evaporate off.

Newer houses (post 2004 apparently) are much more airtight. So far as I have been able to figure out, the problem with these for positive pressure systems is not that the moisture is pushed into furnishings or anything, rather that with no cracks etc. for the overpressured air to escape, the pressure pushed by the fan (which is by design very low) will be equalised by the room. You probably wouldn't get backflow but you're not getting any drier air pushed in. So it shouldn't make anything worse, but it might not be very effective.

Disclaimer: I am not a home ventilation technician. This is what I have gleaned from reading and thinking about this. Why do I read and think about this sort of stuff? I suspect there may be something wrong with me.