On most GPS units POIs are just notes that show you things on the route, but are not taken into account when actually planning the route. Lot of them will let you download and use (like from POI Factory) but they are just alerts at best.
Avoiding low bridges etc when creating the route is something that has to be built into the route-planning software, the same way most units let you set preferences for toll roads and such. Usually only found in trucker- or RV-specific units.