..sunspots, military degredation of GPS accuracy, RAIM issues...
Most GPS approaches are overlays, so it's nice to put the VOR. NDB, ILS into the back ground as a back up..
Do a few stand alone GPS approaches where you don't even have cross radials to double check where you are on the approach, then we can talk about WAAS accuracy.
That said, few pilots are willing, or able, to take the planes down to 100 feet in the soup....so it's accurate 'enough'.