Ok, I wanted to check something. Here's what I found:
At first, I thought "wait until any jobs started..." wasn't working when I tried— that's why I didn't mention it in the previous post. Then I realized something and here's what I think is happening:
Some plugins have a confirmation to update the library. (According to
this post it's a Calibre requirement to prevent db collisions/conflicts.) This confirmation uses a
self.gui.proceed_question() dialog, which is the type that pops up in the lower right hand corner with yes/no/other options.
Count Pages, Extract ISBN, and FanFicFare are the three I know of and tested with. Calibre uses it for a few things too, bulk metadata downloads is one.
When I tested: Action Chains waited for the jobs to finish, but did
not account for the confirmation and immediately fired off the next action.