I have a badger dual action airbrush and compressor I used when I still did aircraft models. I can't think of a single occassion when I have used it for figures, not even for dull coats. But I know a guy who does amazing figures in 54mm, primarily with an airbrush so anything is possible.
I have an Iwata Eclipse I use it alot especialy on skintones. I have it set up so I can get down to a super fine line. I use it for all scales of figures and busts.
I've had a Paashe airbrush for years (a hangover from my early AFV modelling days) however I haven't used it since switching to figures many, many years ago.
I'm tempted to dust it off now that I'm painting the large scale naked Army figures, but I prefer the effects I can achieve with a brush.
I wanted to vote 1 and 6, I own a Devillebis Aerograph 63 that I use for everything from faces to clothing to groundwork, one of the best bits of kit I own.......