Flex Fans:Are they dangerous??

Any fan is subject to failure. Years ago I learned a lesson. I had a OEM 7 blade with one blade bent a little on the end so I just straightened it out and ran it. A few months later I was all under the hood trying to pinpoint this new vibration. What I found was that blade just barely hanging on at the rivets. I thanked the lord for sparing me that day.
Some of the earlier aluminum flex fans were cracking like soda cans. There were some that would throw tiny bits of aluminum also. I'm sure good and bad ones are still available today. Like they say you get what you pay for.
Bottom line, always replace any damaged fan and inspect any aftermarket fan frequently.