Any sufficiently-complex library code likely has plenty of problems, often unavoidably so (e.g. trade-offs between best performance and edge cases). Whether they have been found or not is a function of many, many factors.

> Efficient Market Hypothesis

I've lived long enough to be very sceptical about that sort of thing. Markets tend to be efficient in aggregate, maybe, but on the single case they can fail quite brutally. Look at how "dramatic" bugs are overlooked even in critical pieces of infrastructure like openssl, for years and years; maybe it happens less for openssl than most equivalent libraries, but it still happens.

Also, once the "market" for standards moves on, network effects make it very hard to have any meaningful competition. I mean, who writes XML parsers nowadays? Whichever XML lib was winning when JSON "happened" is now likely to stay in control of that particular segment; and the likelihood that top developers will keep reviewing it, falls off a cliff. Sprinkle a bit of cargo-cultism on top, and "efficient markets" become almost a cruel joke.