I have flown legal VFR in a helicopter with no radio navigation aids at all, in one and a half forward visibility. It's not easy, and you'd better be prepared, with a route you are familiar with, and a current chart, but it can be done. The helicopter gets the lower limit, because, as I learned first hand. When it gets
really bad, you stop into a hover, turn around on the spot, and go back!
I agree that VFR flight with reference to the ground is difficult in very low vis, which is why, for my experience, weather briefings for those conditions will generally include a reference to VFR not recommended. Under certain circumstances, I will do it down to limits, if I know the route and the plane very well.
As for GPS in these situations, unless it is an IFR certified GPS, it probably has a placard somewhere which says that it is not intended as a primary reference for navigation - for this very reason. It can get you into situations you'll have trouble getting out of.
As VFR requires navigation with visual reference to the ground, there is a point at which the ground is moving too quickly for safe "watching where you're going", and it doesn't work well any more. (Cell phone towers com at you too quickly!
)
Knowing when to turn around is a personally developed "limitation", often learned by leaving it too late once!