Yesterday Ulf Wendel created great blog post with deep analysis of JSON UDF functions which I presented at MySQL Connect at September, 21.
Ulf found few bugs in these functions, I reported them all at bugs.mysql.com. You can find numbers in my comment to his blog post. But he also raised concerns, which can not be considered pure bugs, rather feature requests, or even design especiallities.
* First concern, of course, is the documentation. Ulf writes: "Here’s what the README, the only documentation available apart from the *.c[omment]/*.h[elp] files".
I agree single README file is not enough, but this is still Labs project for which I can not abuse MySQL documentation team for making proper documentation for me. But you still can find more