As a general rule of thumb, don't use holy wars when you don't stand a chance of beating all of your target's same-religion neighbours at once. In Iberia you can get a lot of land just by presing de jure kingdom claims (it's pretty easy to become king of Castille, Leon, and Galicia within the first century). If you're not that powerful yet, it might be best to only use de jure and fabricated claim wars until you're strong enough to win holy wars. It's slower but it's much easier.