From my experience, I've only ever used hard shells simply because I've always thought they were made for more treacherous weather. However, I recently saw a video by Patagonia that claimed soft shells are more durable. Is this true? If so, why is it that hard shells are advised over soft shells for harsher weather then?
The hardshell/softshell distinction doesn’t mean pretty much anything today. It used to be that hardshell was completely waterproof and windproof, implying limited breathability, and softshell was somewhat waterproof, somewhat windproof and much more breathable. In other words, softshell used to mean a material you could wear most of the time while being more comfortable than wearing hardshell.
Then some of the outdoor manufacturers unfortunately realized that those softshell things sell quite well and reused the label for whatever they wanted to sell. Nowadays the softshell label can mean almost anything, so that it doesn’t make sense to say if it’s more durable than hardshell. (Especially given that the waterproof and windproof “hardshells” are today often built to minimum weight, so that the toughness greatly varies between different hardshells even within one brand.)
First off - whether one is more durable or not, they're somewhat different products. Whether you need one or the other is more based on what you're doing than on "durability".
Hardshell vs Softshell
note - these are marketing terms in the US outdoor industry. Apparently they're not standard internationally.
Hard shell jackets offer a high level of windproofing, and low breathability. They're also usually the most waterproof. They're made with heavy nylon / canvas materials. Think a heavy gortex rain jacket. The hardshell material itself offers great protection against wind and rain, but not much insulation. Because they're windproof, they usually don't breath as much as softshells.
Softshells are more like polar fleece jackets, perhaps with a cloth cover- they're very warm, because the material traps a lot of air. They also tend to retain their warmth when wet. Some softshells may have a goretex treatment, and be somewhat waterproof. But they're generally not very windproof.
What they're for
Hardshells are necessary for harsh environments with strong winds. You may also have a "softshell" jacket underneath the "hardshell" outer layer for insulation.
Durability in outdoor gear is a odd issue. My experience is that a lot of gear on the market now is designed to be ultra lightweight, often at the expense of durability. Jackets, backpacks, even the expensive ones just get torn up sometimes. You can find products with more rugged construction, but they come at a weight penalty. I could see how a serious mountaineer (which high end jackets are theoretically designed for) may prefer a light outer layer that will just get replaced after each expedition.