Sorry to ask, but are you sure that a money bonus is all there is to it ?
I remember the quote I spoke of as being much more extensive but it could be that the differences in battle difficulty were part of the description. A simple money bonus could of course affect most of what I listed above but I cannot help wondering why the AI maintains its passive role on medium difficulty even when, with my help, as ressourceful, or even more so than it would be in a hard game. I must also stress the fact that in my last campaign, which were on medium, the AI were near completely incapable of fielding strong armies despite its own finances and funding by the generous player - which were both regular and more than sufficient.