Many of the higher end products will void the warranty if an authorized dealer doesnt perform the install. Andersen and pella could care less who installs their products due to the fact that they sell their products to any former drug addict, alcoholic, or weekend warrior with a pulse.

The high end vinyl manufacturers frown upon that business model.

Can a competent window installer install any brand? Yes. That said, it is generally ideal to use someone that is familiar with a given product, as they each have their own "learning curve" as to what works well and what does not... Many manufacturers will offer factory certification which may or may not be tied to warranty coverage as krypton alluded to.

I believe the higher end Pella windows, the ones only sold at the Pella store, are installed by Pella employees and not outsourced as with their lower quality windows sold by the big box stores. At least this was our experience the two times we had Pella windows installed. Same crew came out 3 years later for the second install. No problems with the work they did either time.