POD prices are based on the amount of paper it takes to print the book. This can be a substantial discouragement for buyers. Books from fee-based POD companies won't be reviewed in major venues. This isn't just because they're widely perceived as vanity-published, but because most major review venues, such as Publishers Weekly, Booklist, and Library Journal, will only review bound galleys in advance of publication. Fee-based POD companies don't issue these.
Delays are possible.
No one seems to know quite why, but fee-based PODs are experiencing a logjam in book production and order fulfillment. There may be extra expenses. Some services that advertise themselves as free charge so much for extras that they're actually more expensive than services that only charge a setup fee. Others charge a high fee for things you can purchase on your own for much less, such as copyright registration. Royalties may be less than you think.