I am about as obsessive as they come.
And, being so, this was a question I asked myself, too. As I started researching; I learned that all method have their limitations.
Scales have
varying levels of tolerance. Some can be rather high (when working with the small units of measure DIYers often use). Greater accuracy (smaller tolerances) can be had; but usually at much greater cost. And, even then, they have their own idiosyncrasies: sensitivity to air currents, vibration, and even temperature changes.
Calibration weights also can have a
wide range of acceptable tolerance. And these are for weights often costing $60 (each) and up. How much tolerance do you suppose might be acceptable in the freebies that are included with a $35 scale?
Then, you still have to include the human factor. How often, and how much, one is likely to over/under pour, is also going to effect precision.
Okay, so scales aren't "all that and a bag of chips." What about syringes?
We still have limitations. Syringes made for medical use (the ones we most often find), can have
as much as a 5% tolerance (for a 1mL syringe; larger syringes can have even higher tolerances).
Then, again, there is the human factor. Is the syringe being used correctly? Is the appropriate seal ring being used to line up with the graduation mark... or is it above, below, or even in between the necessary graduation? And, if so, how much? Are there air bubbles? Is the user "double pumping" the syringe (depressing the plunger more than once, to get all the liquid out; resulting in excess material)? And, in my case, just plain old lousy eyesight.
Graduated cylinders, and graduated glass/plastic pipettes, have their own acceptable tolerances, are subject to human error, and... how does one account for the leftover material clinging to the inside walls?
And beakers? Unless they are certified (and then they still have an acceptable tolerance) are marked for "general" reference only. I have a 25mL borosilicate glass beaker, whose silk-screened graduations are fully 5mL off. It is a worthy mixing container; but worthless as a measuring device.
"Well, fine... they all leave something to be desired. Now what?"
For DIY; it really boils down to repeatability, and convenience.
Repeatability is the ability to do the same actions/processes over and over, and get the same results. When I mix the same recipe time after time; does it always taste the same? If it does; one has (in my mind; at least) acceptable repeatability.
Convenience is just that... is it convenient to use? This is where personal choice becomes important. Cost, availability, storage, maintenance, cleanup, ease of use, legibility, durability, etc.; all have varying levels of importance to each of us. Selecting the method of measurement becomes quite personal.
Finding methods that are pleasant, enjoyable, fun... or at least not a hassle; are essential to encouraging the DIYer to mix. If something is tedious, bothersome, or leaves you second guessing all the time; you are going to be less inclined to mix.
For one person; it might be a $1,000 scale, and another $500 worth of calibration weights. While another person gets just as satisfying results; using a handful of 50¢ syringes. Not to mention the near infinite range of folks in between.
Counting drops is not something I can
personally wrap my mind around. Tip size and shape, viscosity, surface tension, and fluid temperature; are all too many variables, to make it a consideration
for me. But, that is my own hang-up. There are those DIYers, who are perfectly content with their results counting drops, and enjoy their creations. Who am I to say they are wrong?
If you are enjoying your process of mixing, and making good/tasty e-liquids; chances are, you are mixing the right way... for
you. 
