Interesting point. My short bakfiets would probably be unrideable at 25kph with more than a banana and me on board.
But if I ever have an excuse I will buy a 45km/h cargo.
If pushed I'd go for the bike with the longest wheelbase and trail and possibly the lowest center of gravity - but I haven't seen any geometry charts for cargo bike with trail figures.
I wouldn't want to ride 40 kph with a loaded cargo bike either.