I have had two sets of Raceface Turbine cranks on different bikes over the past 10 years. If nothing else, they’ve lasted well.
I can’t really comment on stiffness in absolute terms as I don’t have anything to compare them with.
However, I’m around 13-14 stone and am a ‘churner’ rather than a ‘spinner’ in my pedalling, so probably stomp the pedals in an above-average gear which I guess would mean I’d have noticed if they were flexing a lot.
I’ve been happy with them.
I assume the cost relative to Shimano or SRAM is why they’re not very common, plus the fact they don’t seem to be fitted as OEM to much.