Most difficult to fire in terms of skill

Of the three most common source of fuel, coal, wood and oil, I would imagine coal is the most difficult to fire in terms of physical effort.

Wood is challenging based on the type of wood and how dry or wet it is.

Oil would be the easiest physically I would presume, but of course the obvious is often wrong.

All individual steam engines have their own quirks, making firing them different within a single class of locomotives.

I am curious to know which type of firing is the most difficult in terms not of physical labor, but skill. Is it more difficult to fire an oil burner than a coal burner?
That sort of thing.