Hi @cameron.orourke - I'm not sure if there are specific functions that won't work in a field function. It is simply about operating on the current field using the 'this' variable - you can't use a field type as a formatter for data from elsewhere.
There are some Tutorials (https://www.youtube.com/playlist?list=PLKMZcxOsC3u0Y-4CHg5SDpVjTcrvGttTt) and FAQ (https://www.mockaroo.com/help) and the forum search is pretty powerful for finding any issues that have been answered.