In my campaigns the Spanish AI ALWAYS sucks. They never manage to gain even one single rebel settlement. Usually, they are wiped out by Portugal around turn 50.

Probably, it's got to do with them wanting to conquer Valencia, which they never manage. Soon, their economy is crippled and they don't stand a chance.

I find that rather annoying, because I expect Spain to grow stronger and be a worthy opponent in the late game. In my last HRE campaign, I gave them 5000/turn. But what did they do? They took the money and became the richest faction, never even thinking about fielding an army and expand.

Damn AI! Move it!

I think the different development in multiple campaigns depends in a large part on where rebels spawn. The faction whose income is crippled and which has to fight brigands with its armies will have a hard time facing a better off enemy.

For example, France could develop into a powerhouse soon, but if there are brigands, Milan, Portugal and England will beat them to key rebel provinces.