I'd be interested in theories or facts on the mismatch issue.
Some time ago I think I read about it being a good idea for CSS rim on rear and normal on front.
Times and thoughts change.
I think the reason for CSS rim/rear with normal rim/front is that sometimes the CSS rim doesn't brake well in the wet, so with the normal front at least you can stop.
My Raven Tour predates this idea, and has CSS front and rear. I have not yet had any problems braking in the wet.
As far as mismatch front and rear goes, with the same brake type you already have a sort of mismatch, because the front is always much more effective. So having a front rim brake coupled with a rear hub brake (drum, roller-brake or coaster) has never bothered me.
I do warn visitors to try out the braking on my large visitor bike when they first ride it, as the feel and different performance of the rear roller brake can be disconcerting if they are not used to it.
Why have a hub/disc brake at the rear? One possible reason is that hub/disc brakes are less affected by wet conditions. Another is that you eliminate rim wear - instead you get disc pad wear, brake-shoe lining wear, etc. Yet another is that the transmission stays cleaner with no spray of wet muck off the rim, no longer an issue on bikes with a Chainglider.