I've known many families with special kids where the added stress was at least a partial cause of the parents getting divorced.
But another factor may cause marital stress unnecessarily. In our culture many people consider children to be status symbols. Having kids who make the team, star in the class play, get good grades in school, and can be expected to get good jobs as adults makes the parents feel valuable and having kids who may never be able to achieve any of those things makes them feel like failures.
That's a big flaw in our society because children don't exist to be shown off.
One of my college professors gave us this definition of love: "When the happiness and well-being of another is essential to one's own happiness and well-being a state of love exists."
If parents love their children and do the best they can to meet their needs they are successful parents.