A sub panel close to the main service panel/disconnect yes it would be hard to define a safety problem other then the fact that loosing the neutral ahead of this sub panel will put 120 volts to all that is grounded to this panel but the same could be said of every main service panel except that in many cases there would be a water bond that would limit the voltage if the water pipe is also connected to the neighbors house neutral, a sub panel would not have this back up, but a sub panel located 20 feet or more from the main would also have a voltage drop issue because the voltage drop of the neutral will be on everything grounded to this sub panel, now we might not think having 2 or 3 volts voltage drop to be dangerous but lets say that there is a hot tub or swimming pool fed off this sub panel, well it only takes 1 volt to prevent your mussels from being able to get yourself out of a body of water when you are submerged, and very little more to prevent you from being able to maintain flotation or keeping your head above water, and the fact that using the neutral as a ground after the main service bond will also allow much higher voltages if you happen to be standing upon earth or touching something that is at earth potential while touching a grounded electrical appliance at the time of a fault to ground or neutral happens, the fault current can be in the hundreds of amps and the voltage drop then produced on the neutral will be much higher and would also be on everything grounded to this sub panel, so this is not just because some thought it was allowed way back when, because it was never a accepted code practice, it has always been against code to use the neutral for grounding after the MBJ except in a couple areas (ranges and dryers) and remote garages to which both have been removed from being allowed.
but if you want a simple way to explain to a home owner that this should be address is many insurance company's who have their own inspection can refuse to insure property if it doesn't meet code and considered unsafe, and also if they plan to sell the property in the near future let them know this will be flagged by a HI and can possibly stop a sale or at least cause the buyer to ask for a lower price.