You pay instead of getting paid, and you pay a lot. With a traditional publisher, you sign a contract and receive your advance and/or royalty payments. With a subsidy publisher, you pay for all the publishing costs, which can easily be over $10,000.
You risk having a poorly edited book. Editors can be a pain, but they do help. Even the best writers have their works edited, and many new authors need it done. You might think that you know exactly how your book should be written, and I can understand you. I am an author myself, and I disagree with many of the modern-day editing rules. My take on it is that editing is often a matter of opinion: this person (or group) thinks this would be the best way to have written a certain passage, and that person (group) thinks that. There is no such thing as a perfectly edited book. Where one editor has done his best, another might disagree and point out things that he thinks should be changed. With all that said, editors often see things we authors don’t see, and help to improve the book. I have read some works of new authors that were, in my opinion, very good and did not need to be changed in any way, but I have also seen some that made me think, “Boy, if they publish it like this, the book has no chance…” So, if you are a 100% sure in the quality of your writing, then go ahead, but if you are not, you might want to consider hiring an editor. Many subsidy publishers offer editorial services, or you can do some research and find one yourself.
You will have to do all the marketing and promotion. Traditional publishers have marketing departments and sales people who will work on promoting your book and placing it into bookstores. With a subsidy publisher, you will have to take care of that yourself, and you might end up with a garage full of unsold books (I bet you’ve already heard this phrase before). Sometimes subsidy publishers offer some book promotion and distribution, for which you will have to pay extra, but generally, you need to remember that the subsidy publisher’s job is to print your book, not sell it. The seller is you.