I used to use a full harness leash on Floppy and the rest before I had a house. I used to take Floppy everywhere, even into the woods, and never had a problem. Then one day after taking him out, I came home and got him out of the car. Another tenants were baby sitting their 2 year old nephew. The kid made a move towards Floppy and screamed, and Floppy was free of the harness outdoors in about 3 seconds. He ran around the building at full speed, and the only thing that stopped him from running out into the road was he turned wrong and ended up in a corner formed by two walls of the house.
The problem wasn't that the cat (Sherlock) came out of the leash, but he saw a bird and starting straining at it... and then my mother learned why you should probably NEVER use a collar leash. His eyes were bugging out. Fortunately, he survived the experience and no other cats have been walked for the past 20 to 25 years or so. Let's not strangle the kitties!