They are death traps.
A monumentally stupid idea. Think about it for a second, motorway designers put the hard shoulder there for a reason. The government has decided to use them as live lanes only because it's cheaper than building a new motorway, not because they though they were a good idea.
If you have a breakdown and can't get to a refuge area, you are in serious trouble. Your only hope is to literally run from the vehicle, jump over the crash barrier (if there is one). Some stretches I've passed through just have a twenty foot concrete wall, trapping people on the carriageway.
If you have young children who are strapped into child seats then the chances of anybody getting out alive are slim. That's not an exaggeration, the vehicle following yours may well be travelling at 70mph, with literally no way of avoiding a collision. Anybody sitting inside, or standing next to that broken down vehicle, has almost no chance of survival.
That's how serious the situation is. The official advice, if you break down on a smart motorway and can't get to the refuge area, is to call 999. Yes, that's how serious the situation is, 999, defined as an immediate threat to life or property.
A dual carriageway isn't the same situation as the volume of traffic is lower and the speed limit is also lower too. That's why people haven't been killed after breaking down on them.
If you are going to use one then you MUST practice how to evacuate your vehicle. You and your passengers absolutely have to be able to react instantly and get out of the vehicle. You must also practice what to do once you actually get outside. Best advice is to get inside the crash barrier and walk back in the opposite direction that you were travelling in. You MUST move away from the rear of your vehicle, NEVER, EVERN EVER wait next to or in front of a broken down car. If it does get hit by another vehicle, it will be like a bomb going off, pieces of wreckage will fly like shrapnel and you do not want to be standing in the way, wheels, engine blocks, even passengers who get ejected can kill you if you get hit by them. Sorry to sound grim but that's the reality of a high speed collision.
If you are travelling with a partner or another adult and you have children strapped into child seats, decide BEFORE you set off, who will take responsibility for which child. This will avoid wasting time standing in the live carriageway with both people trying to do the same job.
If you breakdown on the live lane of one of these motorways . .
GET OUT. GET AWAY. CALL 999.
DO NOT call from your car.
If you have a dog, leave them in the car.
Good luck.