Writing on Bill of Health, the blog of the Petrie-Flom Center, Michelle Meyer argues that while human subjects "regulations in theory establish three risk-based tiers of review — human subjects research (HSR) otherwise subject to IRB review that the regulations nevertheless exempt; HSR that is eligible for expedited review; and HSR that requires review by a fully convened IRB (everything else) — in practice, the first two tiers tend to collapse into the third. In this sense, and now I borrow from Matthew Stephenson and Adrian Vermeule, IRB review has only one step."
[Meyer, Michelle. “Exempt Research & Expedited IRB Review: Curb Your Enthusiasm |.” Bill of Health, October 22, 2012. https://blogs.law.harvard.edu/billofhealth/2012/10/22/exempt-human-subjects-research-expedited-irb-review-curb-your-enthusiasm/]
I think this implies that even projects that are not federally funded, that don't meet the regulatory definition of human subjects research, or that fit one of the exemption categories are apt to go to full board review.
While it's easy enough to find examples of projects that suffered more scrutiny than required by the regulations, I would note that the exempt and expedited categories remain large. For example, the University of Michigan reports that only 11 percent of projects brought to the Health Sciences and Behavioral Sciences Institutional Review Boards required full review. (Table 4)