Advantages of consequent buck converters
a) Advantage is if you need 3.3V too and the 1.8V cannot handle 12V input. Also any 1.8V converter can be used as it does not need to handle 12V input. Disadvantage is more power losses and heat production in each conversion, and the 3.3V converted needs to be larger to support also the power for 1.8V converter.
b) Advantage is usually that there are less conversion losses and less heat production. Disadvantage is the 1.8V converter needs to handle 12V input, so it might limit your choices but not a lot. If the voltage difference would be larger, it would be more difficult to find a suitable chip that works with given input voltage and is capable of low enough duty cycle.