Think of the disc system like the chargebar you use on a MEC when reloading shotgun shells--it's the same technique.
And actually, the powder measures use a cavity in which you, the operator, set the exact volume of powder to dispense with each pull of the handle. The disc system works the same way except that the disc openings are fixed--meaning that you can't "tweak" each charge, but instead may have to accept a tenth of a grain more (or less) than what you're after with smaller loads, and maybe two or sometimes three tenths of a grain with the larger volume openings.
On one hand, it's not a precise system in that you, the operator, can manipulate to the exact tenth of a grain the charge you're desiring to load. On the other hand, when you get compatible powders and charge ratios figured out, those loads never vary--because they work exactly the same as any other powder measure.
I don't use the disc measure for all of my reloading--in fact, I probably use it only for about half or so. The rest I use with a powder measure and then augment, when necessary, with a powder trickler.
If you have the chance to see a disc system work firsthand (not on an internet video), I think you'll be pleasantly surprised at the simplicity and reliability of it. Until then, as both you and I know, our OWN confidence in our equipment and techniques reign supreme. The Lee powder-through expander dies allow a funnel to sit perfectly in them, so it makes precision charging a case fast and easy.