Very interesting, I really like the idea; you might be onto something very exciting here.
I'll start by saying that I've never played this type of army, so everything I'm about to say is either theory or experience from facing pure cavalry.
While I agree with the other posts that you could use more Might and that you're probably too low a Defence, you can work around that as you get more comfortable with the army. My big worry is that you're putting all the responsibility on the Dragon. You have cavalry and they can make a big impact, but I feel like they'd be too easy to defend against with the army as you have it. Wild Wargs are more about distracting and tying things up than they are about causing damage, Spiders can be powerful but you only have three, it really feels like your only threat is the Dragon. It can work, but I'm thinking if there's a way to make it easier on yourself.
Yes, Druzhag is an obvious answer, but he limits your mobility and the damage boost he gives isn't super helpful without extra Might (for heroic combats). What I'm wondering is if you can fit a Spider Queen in there, give you a reliable second point of attack and a really good way to flank/rear your opponent (Broodlings). Maybe instead of the Warg Chief's warband, squeeze a Spider Queen leading as many Fell Wargs as you can fit. You can free up the points for a Spider Queen and 11 Fell Wargs if you drop the entire second Warband and the Spider in the third.
Looking at the third warband, I would probably drop the Throwing Spears and use those points to give the entire warband Shields. I get what you're trying to do with them, but two 5+ (or probably 6+) shots a turn aren't going to do much. If you really want the shooting, look into squeezing some mounted Orc Trackers in there. At least it's three 4+/5+ shots instead (plus, they're cheaper than regular Warg Riders and hit just as hard).
Hope that helped
