Well that they never fought a real field battle is the fault of both sides. Both sides allways wanted to be sure of the victory. Teutoburg is clear. At the pontes longi, the Romans decides to hide behind their camp walls and finally escaped. Than their is Idistaviso which was after Tacitus an field battle where the Romans seems to have the Germans sorrounded with 8 Legions, + Auxillary, + Preatorian Guard. After Tacitus it was an decisive Victory, but directly after that Germanicus had to face Arminius and his Warriors at the Angrivarierwall. Strange is that at this Point Germanicus had only 6 Legions left and as i mentioned before, after that Germanicus marched directly back to the Rhine. If he was so succesfull, why he wasn't even trying to get Arminius in his Hands? The situation is so questionable that some historians even doubt that Idistaviso ever happend, because of the many things which made no sense.
Also ask yourself why Germanicus had to gain his Triumph over Germania? In the end everything he achieved was to lost the manpower of several legions to gain nothing.
It should also be not forgotten that the Romans were allways outnumbering the Germans of Arminius. The most optimistic estimation is that Arminius had 10000 man at Teutoburg, propably less and never more after that. After Germanicus had 6 and 8 Legions + Auxillary it is not that unhonorably in my eyes to not face them in field if possible
