The Waagh! Field reduces the difficulty of working it right. They couldn't get it to work properly until they got their Waagh! Field excluded from it's effects. As it is, it's very unpleasant for non-Ork Psykers to be anywhere near an active one. Also, they're huge power hogs. In addition, you'd need a full hundred to cover the whole planet of, say, Earth's size, and the things are about the size of one of your current generation of Arcologies, which have 200 floors (100 above ground, 100 below). Gellar Fielding a planet is not a simple or easy task. Nothing riles up Daemons more than being pushed back like that, and the Warp doesn't exactly like the effect, if only in the form of disliking having a section walled off from the general chaos (not always Chaos) of the place.
Upside, a lot of their other technology will work alright.