Then there's the fact that pre-Romans (even if we call them "Celtic", which not all of them were) and post-Roman (even if we call them Germanic) were really a collection of interrelated culture, not a cultural monolith, that were each quite distinct - a Frank was not an Angle nor either of them a Goth (and even Visigoths and Ostrogoths each went their own ways), and Burgundians and Lombards and so forth all come into it too...), so reducing it to "celtic" and "germanic" influence erases a lot of distinction.