Zero, sharing is what these forum's are for
. I'm sure there are some companys that may say apply there filler over primer, but Evercoat does not (unless its just glazing putting). If your bare metal is prepped correctly 36-80 grit sand scrathes, pre cleaner used, then a quality filler applied, sand, epoxy prime, sand, primer sealer, base coat, (mid coat for some jobs) and then clear coat. There should be no moisture, and no filler coming off depending on how thick the filler is (too thick and any filler will come off). It sounds like some prep was missed on the jobs you saw this happening on (that's 90% of paint failures, plus incompatable materials from different companies)
I use House Of Kolor paint products (15 years now) and Evercoat products (6 years). Both companies recommend applying filler to bare metal. I've used PPG, Diamont, and BASF products for a few job's as well. Across the pond there are different laws that require different proccedures... such as using water based products as much as possible (except clear coats)
Personally, why would I want to change the way Jon Kosmoski (founder-painter) of House Of Kolor explains how He's done it for 50 years in several book's He's published about Paint and body work
.
I'm not arguing... just stating facts