If you want the ideal flexing boot (for any style or condition), it ultimately has to match your weight, the range of motion in your ankle/Achilles tendon, and what stiffness you are used to. If your boot does not match any or all of those, you will have problems. The worst problem to have is a boot that is too stiff for you, as you will be constantly forced into the back seat and you'll be fighting the boot all day long. You solve this problem by getting your boot softened to the exact flex you need. If a boot is too soft for you, it will be unresponsive, cause you to work harder to make the same turn (or butter or press etc), and risk more shin bang since your leg is constantly moving back and forth. You solve this problem by stiffening your boot by adding rivets/screws that fix the cuff to the shell (far less tunable than softening a stiff boot).
For sure there are other problems associate with each extreme but the right flex allows for some forgiveness, but is responsive and stiff enough to do what you need it to do. And ideally, this appropriate boot flex should be selected based on your weight, range of motion, and what you are used to, not where you ski.