Polyurethane (PU) has been the standard ski boot shell material for decades because it simply works. It delivers consistent, predictable flex whether it's a warm spring day or bitterly cold, and it transfers energy from your leg to the ski exceptionally well. That's why most all-mountain, frontside, and race boots still use it.
PU is also incredibly durable—boots with PU shells tend to last many seasons without the plastic breaking down or losing stiffness. The main trade-off is weight. PU is heavier than newer materials like Grilamid or Pebax, which matters more if you're touring uphill for hours.
If you primarily ski at the resort and value reliable performance and longevity, PU is hard to beat. It's especially great if you ski in very cold conditions (below -15°C), since it won't stiffen up like some lighter plastics do.