(Disclaimer I haven't read the current docs, just writing this to keep track of it)

On moodle.org, it seems one problem experienced was caused by one of the previous 'cleanup' jobs never running due to another bug with the 'delete unconfirmed users' job (MDLSITE-3046). Upon upgrade, this task is now getting run because of the new task scheduler. This is a good thing, but it killed moodle.org because of the amount of work it needed to do and you can imagine other variants of this same problem

So we need to document this clearly (general problem) in the release notes.