BTW, the RPM ratings are not guarantees: they are ratings.
I've had 3 stainless flex fans and both suffered cracked blades after a while; they crack at the side of the blade just outside of where they are clamped under the riveted holder. This is where the flexing concentrates so that is not surprising. I never had any issues with a multi-piece fiberglass bladed flex fan.
I've raced thousands of rally stage miles on stock type, molded nylon fans with 1:1 pulley setups and revving all the time to 7500-8000 RPM, with a 3800 RPM minimum (wild cam!). Never a hint of failure. So I'm with Mguner on using a 1 piece MOLDED flex fans being OK.