I suppose a flysurfer (like any kite) can be dangerous without a correctly set up leash, I personally test all my new equipment's safety features in light controlable conditions prior to using the equipment in 'real' winds. I think this is such an obvious thing to do that I can not understand why everyone does not do it???
That said, there is always a chance that equipment failure of some sort can put you in a bad position so the option of totally releasing the kite when you need to should be available. That is why it is important to try and keep a couple of hundred yards of safe ground downwind of you when riding or launching/landing. I also carry a knife (just in case)
It is very easy to test and setup the flysurfer leash to ensure the kite does not stay in the air too long, email email@example.com
if you need instructions how to do this.