How could past STEM OPT errors affect my EB-5 application?

Past STEM OPT payroll issues could create concerns in future immigration filings because USCIS may question whether you properly maintained F-1/STEM OPT status, even if your SEVIS record remained active. However, it does not automatically disqualify you from EB-5, especially since EB-5 is a self-sponsored immigrant category, but you should consult an experienced immigration attorney before filing to assess any potential status violation risks.

Thank you for the opportunity to answer your question. First and foremost, the potential that USCIS finds that you have violated your F-1 status has some bearing on the EB-5 petition but would be a more significant concern to your ability to file for your adjustment of status. Generally, when a person has STEM OPT, they must be paid for their employment at least on the basis of working a minimum of 20 hours per week. Failure to be a W-2 employee when you are granted STEM OPT EAD would render you potentially deemed to be in violation of status in the United States. While this is an important consideration for your maintenance of nonimmigrant status and future ability to file for adjustment of status, it should not impact your ability to file the EB-5 petition.
The focus of the USCIS in reviewing the EB-5 petition is to ensure that the business that is the basis for your petition merits approval of the petition and that the investment funds came from lawful sources. In that regards, any income you earned having worked "under payroll" would be considered as unlawfully sourced income and shouldn't be considered or utilized in the investment that would be the basis for your EB-5 petition filing. As long as your investment funds came from lawful sources, and all other aspects of the case are good, you could still be qualified for the approval of the I-526 or I-526E petition.
Based on the information you've shared about your STEM employment, I do not believe that the USCIS would find you eligible for approval for adjustment of status. If you were to apply for adjustment of status I believe that your application would ultimately be denied and you would need to consular process for the approval of your green card, so given these facts, you would need to consider filing the I-526 or I-526E as a standalone petition and plan to depart the United States to complete your green card case by undergoing immigrant visa (also referred to as "consular") processing for the completion of your green card case filing.