Thermal expansion of the window frame being much more than the glass unit can result in extreme pressure on the window seals and thermal spacers. If even micro cracks develop, then the expensive argon or Krypton gas leaks out slowly. And goodbye low u value. I saw in a lab testing site that gave Thermal Expansion characteristics of several materials for 20 degree temperature swings. Fiberglass expanded 25% more, aluminum expanded 300% more, vinyl 700% more. Fiberglass can also form thinner molds than vinyl which can hold more urethane foam and thus can obtain lower whole window u values than vinyl frames (which most leave air channels without using foam).