Haven't seen it in Mail, until just a few minutes ago, on my iPad. Phone has been ok.
Where I see updates not happening consistently is my third-party calendar app.
Some days, updates in background well, others, seems like many hours before it might update itself. Sent a bug report to the developer, got back a response that on the Apple developer boards, lots of discussion on background update is wonky in iOS 8.
Developer also said that iOS 8 background updating is a bit adaptive in that it notices how often and when you access an app, and the OS schedules things when it thinks is ideal. Kinda like OS X process scheduling (do as much as possible at same time to ramp up CPU in a more consistent manner: save battery).
Don't know about that, but sure does look like scheduling algorithms are off. Seeing similar on OS X: Apple's stock launch file for the weekly "periodic" process runs every 3-4 days, even though Apple has specified to run every 7 days.