It's not really much different to what traditional extractor fans achieve. Traditional fans extract the air from your home and send it outside. Nature won't tolerate a vacuum, so as they do this, an equal quantity of air comes in from outside anyway. You probably won't notice it because it doesn't all come in from one place.
PIV achieves the same, in that the air in the home is still replaced with air from outside, but it is achieved in reverse. Air from outside is blown into the property and the equal amount of air from the interior is displaced outside.
Regarding your winter concerns, outdoor humidity actually tends to be higher in the summer, because warmer air carries more moisture. [The reason 'cold' is associated with condensation, is because warm moist air deposits its moisture on cold surfaces in the form of condensation. It's the cold surfaces that present the problem, not the air]
Apart perhaps from the filter, then I don't see much of an advantage of PIV over traditional fans, maybe the air in the loft is a degree or two higher than outdoors to start with, depending on how good your loft insulation is.
Extractor fans / cooker hoods, have an advantage in that they can be localised and remove moist air at source, ie over the shower, or hob before it gets into the rest of the property.
Good ventilation will always help, whether it be from an extractor fan, PIV, or an open window or vent. But just as important is good insulation & keeping the building warm (so that moisture doesn't condense on cold walls & windows) and avoiding creating moisture, such as by drying washing indoors, or taking long showers, etc. Positioning furniture so that air can circulate freely around it can also help so that warm air doesn't become trapped between it and outside walls.
In my last property which was quite small and electrically heated, I installed a HRV system. Fans extracted the air from the property and also brought fresh air in. the two airflows were kept completely separate but went through a heat exchanger in the loft, so that a good proportion of the heat was transferred from the outgoing air to to incoming air. Cost me about £600 but I imagine I saved more than that on energy, and took about 2 days for me to install.
