You're relying on the opponent slamming into your face, and objectives staying central. Due to the size of your arrow units, you'll struggle to screen them both for more than one round of combat, especially if you've been flanked by infantry.
BS spells aren't amazing direct damage dealers (bar the Wurrgog horde clearer), and are mainly buffs. So if your opponent keeps distance and plinks off wounds from your front line your only choice will be to break ranks. Remember the double shooting of the Rukk is in your hero phase, so is quite easy to outrange for the opponent (even before you consider not all of your models will be in range).
There's strength to the list in presence, wounds, and attacks, but you could find yourself quickly overrun.
As an example, a buffed Gore Grunta unit that hits one of your Savage blobs could likely take most of it out if all your buffs don't go off (7-8 D2 attacks a model, followed by battleshock). If they fall, your arrows will be close behind (or they wouldn't be able to shoot due to range, as you have so many ranks) and could quickly be piled in to. This can happen with many infantry or deep striking combos.