Cargo bikes are a great way to transport people - especially kids! For a lot of families it's important for a parent to be able to get two kids onto the bike. But they aren't speed demons. It's not cargo bikes that are endangering people on the streets.
I have a cargo bike that I can get two kids on that has two wheels, and is close to the wheelbase of a typical bike: 135 cm / 53 in. Like most cargo ebikes it's class 1, so speed limited to 20mph assisted. The extra powerful motor is for getting the thing up a hill. It's about three times the weight of an un-motorized bike. It's not that big, not crazy heavy, and not very fast.