I use all three types you mentioned: aluminum clad, vinyl, and fiberglass (actually the Fibrex you mentioned is none of the above - it is a wood/vinyl composite material but not as strong as fiberglass). All three are designed to last a long time, but they have their differences. And to answer your question of "Why do these [fiberglass] windows break down faster than a vinyl one" the answer is "They don't!" I'll briefly touch upon the three types.

WOOD/ALUMINUM CLAD
This is actually my favorite type. Wood windows are strong, they're great insulators, they're aesthetically attractive, and the exterior can be clad with aluminum so that the only maintenance needed is washing the windows. Disadvantages include that it is usually the costliest of the three types being discussed, and it requires painting/staining on the interior side (unless you order them prefinished). Of course those could also be interpreted as advantages. Another potential disadvantage is that if the windows aren't efficient enough to handle the existing conditions, condensation could cause the wood to discolor and even rot. However, modern wood windows are preservative treated before they leave the factory, and more and more are changing to lifetime warranties on the wood. There are different qualities of wood windows, and generally speaking the better ones will last longer than the low-end ones. Some have service departments that make house calls, and some have no service departments at all.

VINYL
Vinyl windows sometimes last longer than wood windows, sometimes less. Like wood windows, there are varying qualities and longevities. Not only are there different qualities of weatherstrip, glass, and structural integrity, there are different qualities of the actual vinyl itself. Some will change color or turn more brittle than others (but they are all a lot better than vinyl products of 25 years ago). Wood and fiberglass window manufacturers are quick to point out how much stronger their material is compared to vinyl, which is true, but you're not buying a slab of wood or fiberglass; you're buying an entire window unit. Vinyl windows, if properly designed, will last just as long as fiberglass and wood. Fiberglass and wood windows are more impact resistant, and have less expansion/contraction, are more scratch resistant, and are stronger/stiffer than vinyl windows, but vinyl windows if properly used don't need to be as strong. How impact resistant does a window need to be? How stiff? Vinyl is just fine for it's intended purpose. I envision vinyl windows being mainly utilized in single and double openings. In new construction, where one might have 4 windows wide by 3 windows high - a wall of glass - I wouldn't use vinyl because it has a lot more flex, and that deflection under a heavy wind load could lead to air and water infiltration. It just can't meet the performance criteria of the others. But that's an extreme case. And even vinyl can come with metal reinforcements inside to make them stiffer. Aesthetically, vinyl can be some of the ugliest windows available, and it can also look quite appropriate, depending on the brand. But beauty is in the eye of the beholder . . . Vinyl window companies usually have superior warranties compared to wood and fiberglass window companies, but it's not indicative of the windows being any better or lasting longer. Here's the catch. Wood window companies usually have a warranty that applies to whomever owns the home. Vinyl companies usually have a lifetime warranty that applies to the first homeowner only. Since people usually move on average every 8 years (at least that's what it used to be; I haven't seen any stats in the last few years), that equates to an 8 year warranty. Most have a transferable warranty program, but the warranty can then be reduced from "lifetime" to perhaps 10 years, and only if you fill out a transfer of warranty form within a certain amount of time, and possibly submit a transfer/filing fee (sometimes for quite a bit of $). So some of the advantages of vinyl windows would be affordability (usually but not always), low maintenance, and very good performance (for the better brands) in medium to small openings (that's my opinion).

FIBERGLASS
There are not as many fiberglass windows companies out there (so far), some of the largest being Pella, Marvin, Comfortline, and Milgard. There is also the Andersen Renewal window, with its Fibrex product it is not fiberglass but instead is a wood/vinyl composite. Fiberglass windows combine some of the advantages of both wood and vinyl. They're stronger than wood or vinyl, have less deflection/flex than vinyl, are more impact resistant, do not require painting/staining, usually cost somewhere between vinyl and wood, have less expansion/contraction than the others, can not rot, and can be painted over in case you want a different color (for example on the inside of the home). Unlike vinyl windows, wood and fiberglass windows are mechanically fastened (which is neither good nor bad, that's just the way they must be made. They don't need welding and can not be welded). As for the Fibrex window from Andersen, I've heard good and bad (like I have with most brands), but have never worked with it and have not closely observed it. Seems OK to me but I don't like their negative selling approach where they slam wood and vinyl. It's backed by a respectable company (Andersen) with deep pockets.

So fiberglass, vinyl, aluminum clad - take your pick. The better brands (assuming properly installed) will perform just fine in all three categories. They have different warranties, some have better service departments, some look better than others, some are better suited for some types of projects than others. It's good to look at not only warranties, options, and aesthetics, but also at performance statistics: U value, design pressure rating, solar heat gain, air infiltration, etc. Again keep in mind you're buying an entire window unit, that has many parts/pieces that are designed to work together - one should not base their decision on just looking at the frame and sash material only (aluminum clad, vinyl, fiberglass). I have Pella windows and Marvin windows in my house - two of the higher end windows around. Both have performed flawlessly and I'm very satisfied with both.

Thanks Tru Blu for the extensive info regarding these windows. It was very helpful and hopefully will help others looking to purchasing new windows.