It'll get done faster than you doing it. The last novel I had ghostwritten was probably 10k words/week. When I write I churn out 500-1k/day and I don't write 5 days a week.
It'll probably be cheaper. Writing a 100k word book would take me ~100 hours. Getting one written is probably $~2k. On the high end that's $20/hr, which I can easily out earn via freelancing or time spent marketing existing books.
It'll be close to your idea but it won't hit all the marks. I've got a grand world building idea. No matter what I present they'll probably miss some of it. Chances are I can go back and re-write/tweak some of it.
If you do a sequel the voice can change. The previous ghostwriter may not be available.
They can bail mid project. I've paid money to one for a book. They start and send a chapter or two. Then I never hear from them again. I can usually get a refund if it's an incomplete submission. But, it's a pain overall.
Quality is hit or miss. I've found some good ghostwriters for a good rate. I've found some terrible ones for 5x the price. Even if you get a sample it could be fake/from someone else. Or they could outsource it even further. I know someone who "Does ghostwriting" for 3-5c/word and they outsource everything to someone doing it for 1c/word. He runs it through one of the automated grammar checkers and passes it along.
Would you mind sharing some websites - based on your own experience or otherwise - where one could find "legitimate" (in quotes because it's, somehow, definition-specific) ghostwriters?
And, perhaps, if you wouldn't mind giving a reference to a sample list of requirement documents (provided by the person who's requesting the work) and, possibly, guidelines for preparing those (e.g., in terms of format etc.)?