Even though I have a few thousand feet of riser cable in buried PVC, I don't disagree with anything TonyR stated above. While the cost difference was certainly a factor, my main motivation was to have the wires color coded. One of several conduits leaving the house carries 8 cables, all different colors. Outside they branch off at various places and knowing which color is for which camera saved a lot of time, and still does when troubleshooting. When I was laying the conduit, I was very careful to keep it clean, with each 10 or 20 foot section verified clean and capped at both ends until attaching it. I also used conduit bodies to keep the pulling distances shorter, plus larger diameter than necessary PVC, mostly 1-1/2 and 1-1/4 inch. Seven years ago, PVC was dirt cheap compared to today.
I've had two totally unanticipated problems:
1. Once a riser cable goes above ground, it totally stinks for UV tolerance. After a couple of years in sunlight the jacket color fades, becomes brittle, and eventually falls off. A fortunate easy solution is to cover the exposed parts with 1/4 inch split loom tubing.
2. Mice get into the conduits if you don't have any open ends sealed. In a lot of cases I just stuff a plastic bag tightly into the open end, in other cases use a PVC cap with a slot just big enough for the wire(s) to pass through.
My thinking at the time was that the labor of burying the conduit far overshadowed the cost of the PVC or the wire, and once the conduit is in, it's easy to add or replace the wires. While that's true, it's still a major amount of work to pull a wire through the crawlspace and a few hundred feet of conduit. I haven't had to replace any cables, but if I ever do, I'd gladly go back in time and spend more on more robust wire. The older body does not want to go through the wire pulling contortions any more.