I've been using these straps for 5 years now with very few drama's.
So far no theft - touch wood. They are a deterrent only!
The Yellow ones are great - you can use one strap for one board!
Some tips I have learnt

Be mindful that if your board is in a board bag or you stack boards and you dont get all the slack out of the straps it is very easy to wiggle the board out buy moving them around. It is the board grip on the roof rack that makes this system work best.
At the same time don't over tighten them, as mentioned it can leave a mark in your board especially if it is hot.
Another common mistake is to have the board out past the feet of the racks. If the strap wraps around the board further out than the feet you can push the board toward the centre of the car and easily slide the board out. Doh!
Great deterrent though so far!