[Q] If my I-829 is denied for some reason, can I take an appeal from USCIS' decision to deny?
Unlike I-526 petition procedure, if I-829 is denied, there is no per se appeal right, but at deportation hearing, USCIS will have the burden to prove by preponderance of the evidence that they were right to deny the I-829 petition. Because you have certain rights in the deportation hearing, USCIS will make sure you did not meet the requirements before denying any I-829s.
Also, if you are unfortunate to be in this situation, and the denial results from a questionable interpretation of EB-5 law by USCIS, then you may have other federal court avenue to fight the denials. At this point, you probably need a good immigration attorney who is knowledgeable about EB-5 law and has experience of litigation at federal courts.
The problem is this litigation path is very costly, although the court decision may force USCIS to pay some or all of the litigation costs, including attorney fees.
No appeal shall lie from this decision; however, the alien may seek review of the decision in deportation proceedings. In deportation proceedings, the burden shall rest with the Service to establish by a preponderance of the evidence that the facts and information in the alien's petition for removal of conditions are not true and that the petition was properly denied.