They are less secure than U-locks, as they will be easier to cut through. But my only bike lock is a simple (Master-brand) coiled cable lock, with a key though, not a combination. My suburban environment is pretty safe, so I am comfortable with that level of protection, and it has been working for a couple years so far.

Apparently it is urban biker chic to have a chain with gigantic (2-3" ea?) heavy links, and a padlock or U-lock to secure the ends, and wear it as a belt, if that strikes your fancy.