The variation from coticule to coticule is one factor and it plays a big part in the difficulty of becoming both proficient and consistent when using slurry. Using the coticule as just a finisher, the only variables (that I can think of) are how fast does it cut and how fine a finish can it leave. But add in slurry, and you get questions like how thick should the slurry be, when should you dilute it, how much should you dilute it by, what stroke/pressure is most appropriate for the work you are doing, etc etc. All of those characteristics are influence by the variation in coticules. (These also apply to any natural hones)
Plus, when you raise a slurry or dilute, you are working with approximations. "The slurry should be the thickness of milk," is a common saying, but that means the actual thickness will vary every time you make the slurry. As you hone, you may push some off, that will vary too. When you dilute, you may go by drops of water, but that will also vary. These affect the consistency of a user on a single coticule. (These also apply to any other hone used with slurry)
Synthetics have far less variables because they are the same. While the exact numbers of strokes will vary from person to person and razor to razor (based on how that steel cuts and how much pressure the given person uses), all the other variables are eliminated.
It doesn't really matter which synthetics you choose, so long as you learn how to use what you get. Personally, I am loving Naniwas. But I have only really tried Nortons and Naniwas (and the King 1k, and Spyderco medium and fine).