No bookstore distribution. Bookstores rarely carry subsidy titles. Unless customers know about your title in advance, however, they'll have no reason to look for it. No library distribution. Like bookstores, libraries rarely invest in subsidy-published books.
Most book reviewers ignore subsidy titles.
In addition, subsidy publishers often send out only a limited number of review copies, and expect you to pay for any additional copies. Most subsidy publishers promise a certain amount of advertising. This is rarely in the place you need it most, however. For example, if your book covers women's health issues, don't expect it to be advertised in health magazines, women's magazines, or other publications that target prospective readers. No editorial screening.