Underwriters Laboratory and Consumer Reports seem like special cases. Don't most private quality control organizations go the way of Yelp, the BBB, the BSA, etc- questionable ratings and bad economic incentives?
I'll go ahead and add Standard and Poor's, Moody's, the SEC, and the FDA to your list of bad examples, since poor quality and bad incentives can arise with either voluntary or coerced funding. But at least with voluntary funding you can take your business elsewhere, or even compete.