Well, my builder used all Flex, directly in dirt. He has been building for 30+ years and says that is what he has always done. I recently dug up some of the flex in which he had run the light fibers for our pool lighting, the stuff looked brand new and will turn away a shovel.

From what I understand (from reading in the old forums) was that the problem with flex was with termites and other critters, not a strength issue. The stuff my builder used is almost impossible to crush. Anyway, we don't have open ground termites where I live in NY (at least not yet), so I'm not concerned.