As you have mentioned, the "Fetch data from page" function will only "overwrite" properties on the target page that are empty (null).
See the reply from Linus Ekström at http://world.episerver.com/Forum/Developer-forum/EPiServer-CMS-6-CTP-2/Thread-Container/2010/5/Fetching-content-from-another-page-in-EPiServer-has-stopped-working/
If I remember correctly you will only be able to fetch properties using out-of-the-box property types - it probably won't fetch custom property types.
(the Fetch Data functionality probably hasn't changed since CMS 6)
Yes it probably hasn't changed much, however there is one difference in episerver itself, it's now possible to create page properties which are value types (int, bool, float) and so on. And they can't be null. So it would be great how episerver behaves in this case.
Current documentation is badly lackinhg in this regard.
I'd like to know how "Fetch data from page in EPiServer CMS" shortcut type is supposed to work. I would expect that it would simply replace all the page properties with properties from target page, hovever that is not the case. It seems that only properties replaced are these which are null. And what happend with struct property types? It looks as if default struct value, for exaple booean "false" is replaced. Is there any docuentation on this?
Editor guide says properties are taken from other page without clarification how.