The best ulock maybe not the best lock as thickness being equal a chain with a good padlock will be stronger than a ulock for several reasons:

-to cut a good ulock you just need to cut the base of the ulock when to cut a good chain + padlock system you need to cut both sides of the padlock's shackle or both sides of the chainlinks
-good padlocks are usually more pickproof than ulocks, actually i haven't seen anyone picking the abloy PL362 but i'm sure someone must have done it somewhere
-because chains are flexible they are harder to deal with than rigid ulocks
-ulocks have only one lock so thieves can easily learn only a few techniques to pick it when chains can come with many different padlocks (but there is more chances that the owner choose a bad padlock too)
-the best ulocks i know stop at 18mm thickness when chains can go up to nearly 20mm
-chains being usually longer allow more choices and then usually better choices when looking for places to attach the bike
-thickness being equal chain + padlock systems are usually heavier and more expensive than ulocks so they are less popular on the street so thieves will have less chances to get used to than equivalent ulocks (but because we usually buy things with price in mind for the same price your ulock will probably be thicker than a chain + padlock system)
-new york fahgettouboudit systems are pretty popular in north america so thieves will have more chances to be trained on than european or other parts of the world's systems and vice-versa