I have no problems with stiffness. I’m only light to be honest (66kg) and I tend to be pretty smooth. I have a set of Racing Zeros too which are bloody stiff, too stiff in fact, but the Strikes are just lovely. They’re fitted to a Specialized Tarmac S Works SL3, not exactly a forgiving frame, but they complement it very well.
It the weather is decent and Im going on a Z1 recovery ride I tend to take the S Works over my steel Genesis Equilibrium because the Strikes are more comfortable than my Equilibrium with alloy clinchers, even when pootling along at 14mph! (if you’ve ever ridden in North Bucks you’ll know that some of the roads here are probably comparable with those of the Gaza Strip…)