Caesar is by far our best surviving source on 1st-century Gallic tribes in general, and especially regarding their relative military strengths. When he says that the Nervii in particular stood out among the Belgae for their prowess, it is difficult to dismiss it outright. Yes, Caesar is writing for his own ends, as all authors, but his apparent goals in depicting the Gauls in any particular way are never transparent. Sometimes he credits their bravery, sometimes their cowardice and avarice; sometimes their lack of tactical sophistication, sometimes their ability to adapt. He paints his own soldiers in similar terms, highlighting their strengths in one paragraph and then their failings in another. His confrontation with the Nervii in the 2nd year of the war was pivotal for him -up to that point-, and it was perhaps important for him to emphasize the implicit danger of the Belgae "coalition" to the SPQR, thereby justifying his current and future actions in Gaul.

For some basic secondary scholarship on Caesar, I would recommend starting with Miriam Griffin's edited volume "Companion to Julius Caesar" (Wiley-Blackwell 2009), which you can find used for fairly cheap these days. It has really great entries by some famous scholars like Badian, Gruen, Ramsey, Lintott, Nathan Rosenstein, Raaflaub, Paul Zanker, and others, and has a fantastic little essay by Christina Kraus on just this topic: Caesar's composition of the Gallic Wars and all the literary/political angles.