Re: Pitfalls of G.fast
There is potential for that device to send 240VAC onto the BT copper pair
There is already that. Just think that pretty well most phone lines these days will have some mains powered kit plugged into them - none of it under BT's control, and some of it from (staying polite) less conscientious manufacturers/sources. And of course, this existing kit will be subject to all the same potential problems (damp, damage, etc).
The other safety concern is having equipment powered from multiple premises - and thus creating a potential for a fault in one property to send a dangerous voltage into another. In practical terms this is unlikely as it would need multiple faults - a "power" fault in one property, a failure of the AC-DC converter in that same property causing dangerous voltages to be sent up the wire, a failure of the isolation on the other end of the wire (inside the BT device, so under their control), a failure of the isolation on another port allowing that dangerous voltage back out again, and a failure of the isolation at the other subscriber's end allowing them to get a shock. That's quite a catalogue of failures that have to happen at once - and having them occur over time and not be noticed is unlikely as some of them would affect service.
There is also the issue of lightning - but that is already a potential issue so I don't think there's any change in risk there.