When you buy one, they come with the warning to drive around first without cargo. But yeah, you need to take it easy. Safe speed is perhaps 10-12 km/h on a bike path with other bicycles, and you need to slow down to almost a complete stop for 90 degrees turns.

Perhaps I should add to this that they're actually super stable at slow speeds, compared to two-wheelers, especially when loaded. My wife prefers a cargo bike to her usual non-cargo bike, I think for this very reason.