Even though Hal is a smart guy. Many of us already realize that there are problems with safety boxes, and surely we do not know the details of exactly how Hal saved his keys in safety deposit boxes, but that could be one point of failure if that might have had been the ONLY place that he had his keys and/or other instructional information related to his bitcoin.
Good point, but the sad truth is you have to store your private keys somewhere. If it's a physical location (be it for paper wallets or electronic devices) it'll always bear a risk of someone unauthorised getting their hands on it.
That could be mitigated by splitting it in 2 or more parts and store each separately. In Hal's case - he referred to one box, but at the time he passed away (2014) nobody really knew where BTC will go, so, I reckon, at that time it was probably not the most valuable asset of the inheritance. And we all tend to apply level of security proportional to the value of what we try to secure.