Fiberglass reinforced vinyl windows -- better/worse/same as being reinforced with aluminum/steel?
Reinforcement is very important for vinyl windows. That said, lower quality vinyl windows always need reinforcement to help with warping. This only minimizes or delays warping due to the low quality. These include all the vinyl windows sold at Home Depot, Lowe's, and other building supply stores. Although not all those are reinforced. High quality vinyl windows won't warp but are reinforced to minimize deflection which is when strong winds hit a window causing the window to bow ever so slightly and allow air to infiltrate. This happens to wood windows, aluminum and vinyl. Reinforcement also allow the window hardware to be fastened to both vinyl and reinforcement. Fiber glass and other composite materials are used as reinforcement because they are not thermally conductive like aluminum and steel.

High quality vinyl window brands are Okna, Gorell, Polaris Ultra series, Soft lite, and sunrise. All of these windows are reinforced. However, I don't think the Sunrise window hardware is fastened to the fiberglass.