The Ulock can be defeated, it just requires different tools then the chain. For the Ulock they usually use 'leverage' tools such as a car jack which attempts to defeat the device by applying enough pressure to have it break open or just prompting the locking mechanism to fail. For heavy chains, large bolt cutters are often used.

This is the reason that for maximum security people recommend that you have one of each. The logic being that a would be thief would have to carry both a leverage device and a bolt cutter to defeat your locks.

Of course, a portable grinder or circular saw with a metal cutting blade will easily cut through both, but those tools make a lot of noise and draw attention.