Hand wash only hip protectors can be a hassle on multi-day ski trips, since they typically need several hours or overnight to dry after washing. If you're skiing for a week and only have one pair, you'll be putting on damp gear by day three—which is nobody's idea of a good morning on the mountain.
That said, some hand wash only models use premium materials or advanced impact foams that simply can't handle the agitation of a machine wash. If a particular protector offers noticeably better protection or comfort than machine-washable alternatives, the extra care might be worth the tradeoff.
The practical workaround: bring two pairs if you go this route, or look for a model with removable pads that you can hand wash separately while the shorts dry faster on their own. Just check the care instructions before your trip so you're not caught off guard.