Are they durable? Well, it depends on several factors.
First, the type of wood they are made of. There are just few wood types that are proven to be really resistant to outdoor elements such as hot sunlight, UV rays, rain, and snow. When you buy wooden furniture, just look for the following wood:
These woods are also resistant to destructive bugs and insects.
Other woods that you can find like pine or birch are not as great as the list above but they can be treated for durability. They can be pressure treated, oiled, or painted with weather and bug resistant coating. These types are cheaper. If you have to buy them, ask the manufacturer or dealer for these characteristics. This may help you determine what to do and what to add to increase the durability.
Secondly, how you took care of them also affect durability. Even the best wood outdoor furniture need proper care and maintenance. Their natural and artificial resistance will degenerate over time. You need to oil them or apply a good protective coating once a year or so.
Any wood, can get their color change or fade to gray. But it does not necessarily mean they get weaker. Its just a natural process of the wood getting dehydrated. It should not make a big problem because you can bring back their original look or make something better by applying oil, stain, and paint.
Please do not forget to buy a patio furniture cover. A good cover will guard your furniture against sun, rain, snow, and dust. Patio umbrellas are also beneficial for your outdoor furniture as they can block hot sunlight and UV rays.
If you take good care of them, you will enjoy your pieces of wooden patio furniture for a longer time.