Die-cast aluminum is brittle too. The magnets are cast into the wheel and pinned on the back so it's unlikely that one would fly out unless some major failure happened.
Resonance isn't what makes an iron flywheel burst, it's centripetal forces that exceed the material strength. In other words, spinning it too fast. The only way to know if the aluminum ones are safe to the claimed RPM is to test them on a spin fixture or wait until lots of people are using them and see how many explode in the field. Keep in mind that Chinese sellers claim whatever they want because there's no way for you to sue them after a flywheel explodes and someone gets hurt.
I know it's not $25, but PVL flywheels are a known quantity that it suitable for high RPM engines.
