I run the same flex fan you posted as I cant fit a factory clutch fan in mine and I have also run flex fans for years and never had any blades come loose. I check them every so often when the hood is up. I have used this one since 2006 and its still tight and not loose at all. I dont race alot as my son dont either and we still have the factory flex fan on his Dart which we always check and has also never had a problem. I did install an electric pusher fan for back-up as when I sit in traffic for a long time (about 1/2 hr) it creeps up to 200 and then I hit the toggle switch I have the electric fan on and it keeps it from going over 200 when sitting still in traffic.
Factory clutch fans work on eng temp as if you are racing and the eng heats up enough before you launch the factory style clutch fan can engage some and rob some hp going down the track. To me you need to be sure you leave the line below 180 to be consisdent with a factory clutch fan to be sure its not engaging any when you make a pass. I have heard some aftermarket clutch fans work on rpm only and just slip at high rpm and engage at idle but I have never used an aftermarket clutch fan. Ron