While I am not 100% sure, I would say the main problem is weight distribution. Before long propshafts were feasible, putting the prop in the ass of the plane was not possible because the weight distribution would be off with an engine mounting behind the wing. With wing mounted props (as with wing mounted jets), you want the weight to be on the front of the wing to decrease the tendency of the wing to twist up.
As far as I have been able to discern from previous discussions of pushers on this forum, they are theoretically as efficient as pullers.
Problem #2: Air AFTER the plane is more turbulent. Pullers get less turbulent air and so can be more efficient. I'm not entirely sure about this one.
Problem #3: At least in smaller fighters, the pilots don't enjoy bailing out only to be chopped to pieces by their own prop.
The German plane you are referring to is probably the Do-335 Pfeil. Yes, it had a pusher prop and rear engine, but it ALSO had a nose prop and front engine. Very cool plane http://www.aeroflight.co.uk/profile/d335top.htm
. The Pfeil resolved the bailout problem by having a jettisonable prop and fin.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo