Off the shelf / cut-down products are typically made to be disposable and should not be expected to wear, function, or look as good as a custom product, which is made to fit each window opening.  They are also made of inferior materials.  A good example is the plastic and vinyl used to make them will not hold up well to Ultra-Violet (UV) exposure, causing it to break down and become brittle over time.  Custom products have higher grade plastic and vinyl materials and will last longer.  Also, parts can be hard to come by for off-the-shelf / cut-down products sold at big box stores. As a result, when a simple part breaks, it’s time to replace the entire product.