Yep they are perfectly safe if used correctly,.and a bit more care and alertness is practiced.
They become dangerous when you chuck the idiots into the equation, the careless and apathetic car drivers,.and the same type car drivers who have managed to blag a truck licence, and.who are driving their trucks like their cars.
The ones who refuse to use them properly, in favour of staying in the wrong lane, for whatever misguided self righteous reason, are equally as guilty I reckon also. 
My youngest daughter, her partner, who doesn’t drive, and their 5 yr old and toddler went on their hols on the M6 last week, I made her make sure that they had a plan in place to get everybody out asap if ending up on the hard shoulder, or worse on a smart, perfectly safe if everyone else on the road is alert, …but again, ‘‘the idiots’’.