You pay instead of being paid. Yes, POD is much more affordable, but you still pay. The cheapest POD publishing packages usually have limited distribution and marketing options, and I don’t recommend to save on that. After all, that’s the whole idea of self-publishing: you publish your book in order to get it into the readers’ hands, not to sit around without being noticed.
You risk having a poorly edited book. (See above.)
You will have to do all the marketing and promotion. Even if you choose the best publishing package, most of the work of promoting the book will be yours.
POD Stigma. Unfortunately, there is some prejudice towards POD-published books. Some reviewers refuse to take them, and many bookstores will not stock POD titles. The latter has a reasonable explanation: many POD publishers do not accept book returns, which means that if the bookstore has some unsold copies left they cannot send them back to the publisher. This is a serious drawback. Stores need to be able to return unsold books, and good POD publishers are now providing this service. But the sad thing is that the stigma is still there. A bookstore manager will often look the book up in their computer, see it listed as a POD title, and refuse to stock it “because POD’s are non-returnable,” without even checking. So if you choose a POD publisher you will need to: A) Make sure they accept book returns; B) Make sure you tell that to each and every bookstore you contact.