I've never used a gearbox setup to make furled leaders, so I can't speak to how the ones I make using Kathy Scott's method compare in terms of quality. I can say, however, that every one I've ever made and used has performed exactly like every other one, which was very well. As far as them being a pain to make -- I don't know how other people do it, but back when I was doing a lot of them I could make a completed leader (with tippet ring) in less than 5 minutes.
I have seen some very impressive gearbox-driven jigs, but have never felt the need to invest time or money in making one because I've been so pleased with the performance of the ones I've made.
As far as the spray problem -- spray results from water getting "inside" the leader--that is, in between the twisted (or "furled") strands of thread or mono or whatever. Keeping the strands as tightly twisted together as possible during construction would doubtless help with this, and I would imagine that this is one area where the superior precision (and resultant repeatability of results) possible with a gearbox-driven jig would outshine the simple leaders like the ones I make using an electric drill and manually-applied pressure during the furling process. My leaders do produce spray if left untreated before fishing. If I coat them with mucilin wax first, I can get several hours' fishing out of one before it begins to absorb water and spray. If that happens, I just switch to a fresh leader. I coil the wet one around the crown of my hat to dry; once dry, it's good to go again.
It's interesting to me to hear how other folks make their furled leaders, and what others' experiences with them have been.