I think the ghostwriting itself of some ‘individual item’ whether it´s a short piece or a book isn´t that difficult, but if it needs to ‘fit’ to other items and shall be ‘seamless’, then yes, as it would require ‘studying’ what else of that kind the author wrote, analysing and applying it, and thus time and money.
As a real example, a fantasy author wrote over ten tomes of a saga, but unfortunately died before the, already planned, last two books. Another author, fan of the series himself, took over and, knowing the ‘world’ by having read those over ten tomes and with the help of the notices of the late author and the help of his wife and editors, wrote the last missing books, but the fans had to wait for years, until they were published, I don´t want to know how those books would have read had they been written within fiverr’s 30 day limit. They have lots of pages too, on top of that.
While on the other side, if you ‘study’ the last week or month of someone´s twitter posts, you probably wouldn´t have much of a problem ‘to hit the right note’ (all that under the assumption you are a good writer or at least not a hopelessly bad one in the first place).
I might be able to start right away to write, say, book 8 of a vampire book series I read the 7 existing books of, because I´m familiar with the style and mannerisms and characters and plots and such and could produce something similar maybe, but without having ‘studied’ other things an author has written, I could only write based on what info/instructions I´d get of course. I suppose the ‘ghostwriting market’ is pretty varied, depending on what the customers for ghostwriting services want, demand and offer. If you´d want someone to write in the same style as you do, you´d need to pay them for the time they´d need to ‘learn’ what your style is’, for example, as far as I can see it, or they´ll end up with whatever you´ll write else.
Very interesting topic again to think and read about lol. I´m glad with translating and writing myself, I think I´d have problems with ghostwriting. Must be difficult, if you want to do it well.