The idea behind the motor mica is to be a dry neck expander lube that won't have a chance to contaminate powder or grease the bullet. If you tumble your cases in plain corncob or take other steps to clean the lube off after sizing and before loading them, you don't have to worry about contamination. In that case, use what works smoothest, and the regular case lube will probably be it. Lee's case lube is an automotive assembly lube, and it does pretty well in case necks and you can rinse it off in water or simply let it dry later.
There are other alternatives to motor mica. NECO sells small jars of ball bearings that are tossed in moly, and you set the neck in it and give it a twist to pick up a trace. That seems to be all it takes. Imperial (now Redding-owned) sells a small tin of powdered graphite you just poke the neck into. Either works better than the mica, IME.
As to drawbacks, you'd need to do some experimentation, but I expect any lube, wet or dry, that is not removed from the case neck will reduce start pressure at firing at least a little. It's doing the opposite of a crimp in that situation. Moly coating bullets does the same thing, but even more so. The concern would be that if the lube were irregular in any way, it might have an adverse impact on velocity extreme spread. On the other hand, if it prevents cold welding of the bullet and brass over time, it might actually improve consistency in ammunition that is stored for long periods. I can't say I've ever noticed an overtly adverse effect, so this is pretty strictly theoretical, and not something I've measured.
One place that motor mica is useful is in bullet lubrication. Hornady's swaged bullets are waxed and coated in the mica, and at moderate velocity that seems to work well. I, and a number of others, have taken to sprinkling motor mica over our tumble lubed cast bullets when the Liquid Alox (or Xlox, depending whether you use Lee or White Label) is still tacky, then rolling them a bit. It makes them easier to handle and reduces lube build-up in my seater die ram. Leading reduction as compared to the liquid lube without mica is, again, not something I've studied systematically, but it sure doesn't seem to hurt.
