A cable lock can be broken easily by someone with the (quiet) tools.
Chains, even the hefty motorcycles ones, are more generally vulnerable than a u-lock. Part of the reason is that they can be put on the ground for access with bolt croppers. Sometimes the padlock is the weakest component.
A half decent u-lock with little space to insert a bottle jack (the tool of choice for these) is as good as it sgets. I still maintain that my relatively cheap u-lock is not a great deal more vulnerable to most tea-leaves than the £70 ones.
Having said that, a cheap cable is still better than nothing.