Since I started really tightening the prop nut, I've had zero problems with slippage, and I've used both Mejlik and Biela carbon fiber props. It's helpful to use a spinner that has good knurling on the backplate to hold the prop (like a Tru-Turn). I use Dave Brown spinners that has very shallow knurling, so I also fold a piece of sandpaper in half with the rough side out and maybe 2" square, and put that between the backplate and prop and tighten that down with a socket wrench (TIGHT!).

ZERO slippage.