The Unioners are the least ideological of the Rheinland unlawfuls (at least in Disco and I can recall something like that in vanilla lore as well), they are more than the other just pure pirates. The Bundschuh are ever since vanilla, the most ideological ones. In Disco this over the years has grown to affect the relations between the two significantly. The VWA's stance is that the Unioners are nothing but selfish bandits and brutal pirates. Also note that the Bundschuh are now also at war with the Corsairs due to a Sair npc base being placed in Frankfurt.
But yeah, Bundschuh - Unioner feud started long ago. I remember hostilities back in 2011 between VF and AGS-U already and honestly, I think it's a good thing. It's just one more thing to differentiate between the various unlawfuls.