In case of a tyre blowout or engine failure it is usually possible to pull over to the shoulder, but it may well not be possible to continue another mile to the next refuge layby. There have been some nasty accidents on all lane running smart motorways where a broken down vehicle stopped in the left lane has been hit at 70mph. These would not have happened if there had been a shoulder. In such cases safety is dependent on a human control room operator noticing the incident on CCTV and turning on the signs to reduce the speed limit and close the blocked lane. There is a window of vulnerability before this happens.
Very few 70mph A roads have 4 lane carriageways, and they usually have soft shoulders where it is possible to get at least partly off the carriageway in an emergency. This is not the case on a smart motorway.
There are not always soft shoulders on A roads. I think the thing about an A road is that people expect that someone might be stopped in the nearside lane. This would also be true of a "permanent" smart motorway.
My
suspicion, though I've not seen the stats, is that the ambiguity of part time hard shoulder running poses a specific risk[1], but with permanent all-lane running that ambiguity doesn't exist, and so I reckon it might well be the case that the risk posed by a vehicle stopping in the running lane (very rare these days) is outweighed by the risk to a vehicle on the hard shoulder by an inattentive driver hitting them - and so all-lane running isn't actually that dangerous.
Variable speed limits on the other hand demonstrably work[2] by preventing the "brake light cascade" effect causing congestion. Some of the M25 has this alone (with a conventional hard shoulder) and I suspect this will remain and be rolled out further. I'd imagine, now the cameras are on when NSL is in place, these also reduce accidents and the severity of accidents by keeping most traffic to 70mph or thereabouts.
[1] A few risks, actually. One that people might think it's not in use as a hard shoulder when it is and drive in it and hit someone, or vice versa. But also the risks posed by people choosing not to use it and thus other people "undertaking" them when not expected.
[2] I was commuting by road from MK to Slough for a year or so, and during this time I experienced the M25 without it, during roadworks and with it. It was
noticeably better in the morning peak with it than without.