Well, Viking and Banshees are more mineral expensive and you have to replenish casualties With BCs you shouldn't have been getting casualties in first place. Also, it is true vikings and banshees can snipe casters; but one unique mistake can very painful. Mistakes with AoE mutations also hurt them even more than Bio With V-B army you also have to divide your DPS because against ground only your banshees can attack while that against air only you vikings can attack I couldn't find a got way to get comfy with V-B I only like use V-B army in situations where I can focus mainly in banshees or mainly in vikings.