Pigeons of all varieties do make rubbish nests, as a rule, some particularly rubbish. But what they lack in architectural or engineering acumen, they make up for with sheer bloody persistence - they'll keep on and on trying, often right into the winter. Stock doves and feral pigeons, though, usually find some solid ledge or cavity to build in; woodpigeons just fling a few sticks any old where and hope for the best.
Besides being structurally poor, the nests are often very obvious as well, and while nests like Andy RM's might be inaccessible for mammalian predators, they're no problem for crows, magpies etc. It was just a matter of time really, before an egg fell out or a corvid got them.
(I'm a chartered wildlife biologist and as it happens I'm currently working in the field on a big nest-monitoring study).