Windows have to tick a lot of boxes to make them fit for purpose.
When comparing technical parameters, the basic point to remember is that there’s a big difference in performance between the glass on its own and the overall window with frame. It’s usually the frame, especially where it meets the glass, that is the weak link.
Most window companies now quote overall U-values for units in frames (not a mid-unit value, which is better). Always make sure you are comparing performance like with like.