Drop means you take fresh tissue and just place it in PFA and let it sit. The advantage is its very easy and you don’t need to do perfusion, which can be challenging and messy. The potential disadvantage is that it takes time for PFA to perfuse from the outer surface of a tissue piece to the inner core. If you have a large sample, like a rodent brain, this will affect your results. Outer layers will be fixed faster and longer than inner core structures. This will inevitably lead to variation based on location.
Thus, for brain, most people prefer to perfuse. For spinal cord and DRG, I think immersion fixation is adequate because the tissue is small.