Writing as a TurboTax employee, we generally don't reply to questions such as this, where you are obviously seeking community input from other Oregon income tax filers; and not looking for legal or technical answers to tax and software issues.
However, I feel compelled to weigh in on this particular thread, because I have a relevant professional experience to share with you. As part of my practice as a CPA, I prepare individual and business tax returns for a small private group of income tax filers, in addition to my employment at TurboTax.
With that fact understood, I had reason to prepare and file an Oregon Form 40 (full-year resident) state tax return last season (2015 tax year) with the Oregon Department of Revenue. It was filed in late March, and subsequently "held for manual review" as you indicate in your text. The taxpayer at issue did not receive their state of Oregon tax refund until mid-August, months later that same year.
There were no adjustments made to the tax return by the Oregon Department of Revenue, no questions ever asked of the taxpayer (by letter or otherwise), and no other explanation ever provided, aside from the indication that it went through a manual review process. The Oregon Department of Revenue did pay a small amount of interest (as required by Oregon law) for the delayed refund, in addition to the original refund claimed.
Thus, although it did take nearly 5 months from the time of filing, the taxpayer here did eventually receive their state of Oregon refund, plus interest.
I would invite others to post their personal experiences as well, as I have just this one single case to share.
Hopefully this story will give you one indication of a successful (albeit slow to resolve) outcome, where a manual review of a tax return is performed by the Oregon Department of Revenue.
I filed January 29th, and I'm still under manual review, and it feels hopeless, honestly.