The disadvantages of caster sugar are the same as the disadvantages of granulated sugar or other forms of sucrose as caster (0.5mm) is a more finely ground version of granulated sugar (0.35mm). Caster sugar is used primarily in cake mixes so that the sugar dissolves more efficiently into the rest of the ingredients.
The disadvantages of using caster sugar
A teaspoon of caster sugar weighs more than that of granulated sugar simply because more fine granules can be packed into the space.
Therefore one could be at a disadvantage if one misunderstands that they are putting a greater amount of sugar in their tea for example than they thought. However, this difference is extremely small. The way to avoid this problem is to measure the sugar ingredient by weight instead of with a teaspoon or other instrument.
If one is to look at the disadvantages of using caster sugar in general one could say that using refined sugar or sugars that are not naturally occurring in our diet can be detrimental to our health on the whole.