A: It's usually less accurate.  Let's say you want to make 100 mL of a 0.0001 M solution from a 1.0 M solution.  If you did serial dilutions, first you would dilute your original solution 1:10 to make a 0.1 M solution and mix well to evenly distribute the solute. Then you'd dilute this one 1:10 to make a 0.01 M solution, and so on. 

However, If you wanted to make the 0.0001 M solution direction from the 1.0 M solution, you would have to dilute your original solution 1:10,000.  What is 1/10,000 of 100 mL? Only 0.01 mL!! Even if you had an instrument capable of measuring this volume, it is so small that your possibility for error and uneven distribution of solute in this tiny volume is high.

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