My experience with it has been that each device has it's own backup. I get this from messing around with my Nexus 7 and my Galaxy S II. Other than that I do not know.
Apps:
My Nexus 7 has these apps Caustic, DC Comics and 20 Minute Meals that upon factory reset of my Galaxy S II are not installed on the Galaxy S II.
My Galaxy S II has theses apps DriveDroid and Human Japanese that upon factory reset of my Nexus 7 are not installed on the Nexus 7.
The apps are compatible with both devices, so incompatibility cannot be the reason for them to not be installed on the respective other device.
Data:
As for Wifi and other data I'm unsure as each time I have set up the Wifi on each device during the initial android setup. As to other google accounts that you may have they do not seem to be copied to each device and the same has been true of Skype and GitHub accounts on each device.
