Most investors in the EB-5 program are still in the pre I-829 stage of their investment so it is still too early to say. Because the actual investment horizon (i.e. exit period) can often be considerably longer than the two year minimum requirement for I-829 (5 years is a common time line), it is likely that it will be some time before this question will be able to be answered fairly.
Most of the investors in the past were VERY small in numbers. For a period of years in the 1990s, some very shady characters got involved in EB-5 and created Regional Centers that offered poor quality investments that did not help their alien investors meet the statutory requirements. Many of them filed lawsuits and Congress passed am amendment to help them out. It took USCIS far too long to publish regulations and only did that very recently. Anyone who could have been granted full status got it and the potential denials were placed on hold. IF in that long waiting time they finally made good, they can now try to get conditions lifted. Those who fail to get conditions lifted, may get a new chance and those who can't or won't take that chance will be denied. As for the Newer investors in the resurrected EB-5 Program, it is still too early to tell for sure but the preliminary figures suggest that I-526 and I-829 denials have declined meaning that approvals are up.
Many investors do receive back much of their invested capital, however this is not guaranteed because the investment needs to be "at risk." The "at risk" requirement states clearly that there can be no guarantees about returns on investments. Most people do not get a high return on their investment, but they do receive some of their funds.
Many investors do receive much of their initial capital back, however it is not guaranteed. To satisfy the "at risk" requirement, there cannot be a contract of any kind stating the repayment of an investment. If an investor receives their investment back, they generally do not receive a high return on their investment.