Even with a harness there are two times you won't be secure. Going in/up or coming out/down.
Being safe will get us home most of the time. Still, even the best of preparation can be for nought. The gentleman that taught me the importance of using a safety harness was decades ahead of most hunters. He fell either going into his stand, or coming out. He never said before he passed away. A more safety conscious hunter I have never met.
Stay calm, stay safe, and don't rush when getting into or out of your stand. Hunting is inherently dangerous. We all have to do the best we can so we make it back to prepare for the next trip out.