ROVs and AUVs indeed have the distinct benefit of not killing you when they implode.
So many different ways to die in a submarine, so many failure modes. It's one thing to solve all the engineering problems you can identify. It's another to learn from all the failure modes everyone else has encountered.
If you do find yourself in a situation where you are forced to ride in a billionaire's submarine, just make sure it's the billionaire who is fanatically obsessed with deep sea exploration...