Hard shell knee pads have a rigid external cap over the padding, and they're built for high-impact situations. If you're race training or running gates, hard shells are actually mandatory—the rigid cap spreads impact force across a wider area and slides smoothly over snow and ice when you hit the ground or a gate. Soft shells can catch and grab on snow, which nobody wants at race speeds.
For typical all-mountain or recreational skiing, hard shells are usually more than you need. They tend to feel bulkier and less flexible than soft or hybrid options, and most skiers find them uncomfortable for all-day wear. They're also designed to be worn over your layers, which changes how they fit with your setup.
The bottom line: if you're not doing gate training or racing, you'll likely be happier with a soft shell for comfort or a hybrid for a nice middle ground of protection and mobility.