Ignoring things like aesthetics, personal taste, etc, it's more or less what you said:
you seem to be trading protection against pressure for protection against being thrown around
...but of course you can mitigate the loss of protection against being thrown around by packing strategically to fill the space and use soft items to protect fragile items (e.g. wrap clothes around things, wedge things in place, fill gaps).
Hard cases can bring a benefit when your luggage includes fragile / hollow items that would squish or crush in on themselves if weight is piled on them in a soft case. For example, I went with a hard case when transporting an unusually shaped musical instrument that didn't fit any proper purpose-designed touring-grade hard cases - but I've used soft cases (backpacks, sports holdalls, fabric wheeled cases) almost all other times, with normal luggage like clothes, books, bottles wrapped in clothes, etc.
Don't rely on hard shells alone, however - as people have said in comments, hard shell cases tend to be quite thin and flimsy and crack disappointingly often. I'd recommend taking additional precautions. For example, my fragile musical instrument was:
- Wrapped in clothes to reduce movement and protect it if it did rattle around
- Filling a quite sturdy cardboard box
- With other hard items strategically placed around it to act like struts, plus more clothes to stop the carboard box rattling in the case
- Inside a hard shell case
- Which was stuffed sufficiently full that the shell sides didn't bend or buckle more than a little when weight was put on them (I tested)
- Which I think as a final precaution I then wrapped with that plastic wrapping stuff at the airport (probably doubling the thickness of the flimsy outer shell!)
All together, it worked very well. To be honest, with all of that it may well have been okay in a soft case too... but the hard shell probably helped as one factor among many.
As for waterproofing, there may be a small benefit, but only a small one. One time I wished I had a bag with waterproof sides after a trip in an unexpectedly waterlogged canoe in unexpectedly choppy water, but even then, it was only the outer clothes that were slightly damp. It'd need to be virtually submerged in water for it to soak through layers of clothes in a tightly packed soft case, and most shell cases can let water in at the zip. If waterproofing is a major concern, a better solution is to use dry bags inside whatever case you use, or do the thing where you wrap the case in plastic at the airport.
Under normal circumstances, soft cases are probably slightly preferable because they're easier to pad out and less likely to be seriously/irreparably damaged (e.g. it's easier to fix or patch a tear or stretched seam than a hard case that cracked after falling).