thats absolute bullshit, if your car is a shit box course it will have issues..... blame the radiator not the cap......FFS i can tell you now if it had std cap it would have spewed all its coolant all over the place then probaly cooked itself.... if the rad was in good nick it would been fine. think of it as fitting a bigger turbo to a tired engine.... not a good idea.
the whole point is to increase the pressure to raise the vapourisation point, works very well so well in fact i have them fitted to all the tow cars we use.
When towing the race cars around the old patrol (2006 model) used to vent the cap during towing and continue to pump the coolant out of the resevoir now it is sweet. Same with most cars that see track days or hills work.
best budget cooling upgrade available.