What guarantees can I expect from a ghostwriter?

The only guarantees a writer can offer are those elements in the writer’s hands. This means that a writer can perhaps guarantee your satisfaction with the writing or your money back (though I don’t know if anyone goes even that far). But a writer can of course not ever guarantee publication, unless you are self-publishing.

A writer is in charge of the writing ONLY unless he/she has agreed to help you in other matters. That means the writing is usually the only area to possibly be guaranteed.

The closest thing to an exception that I’m aware of is my own service. I don’t expect my clients to understand the publishing world, so my service gives them EVERYTHING they need to get published. When the manuscript is finished, I work with them to develop anything else that's necessary for submissions to agents and publishers, including query letters, cover letters, and a proposal. I give them a list of all the agents and publishers appropriate for their book. And if they have proof of actually submitting their book and they can't get anyone to publish it after a year, then I actually produce the book for them. Then end up with as many as 1000 copies printed and in their hands to sell.

As far as other writers go, since very few will ever pay money back to you after providing work for you (in other words, if you’re unhappy with the writing), the best form of a guarantee may be to get samples — CUSTOM samples, written for your project — before investing. If a writer will not provide this kind of sample, you should definitely be able to see other samples … and you should be very comfortable with that writer to trust the work without something done on your project.

And if you are only provided samples from other projects, look for one written in the tone/style you’re looking for in your own project so you know the writer can provide what you need. Otherwise, if they want the job, tell them they need to produce something custom with that kind of tone/style. It’s the only way you can really feel a guarantee.