I've noticed that CNs from nations with weaker land power get independence much more often, which makes sense not just because they are more likely to win but because they will declare the war easier. So Norwegian/English/Scottish/Dutch/Breton colonies seem to break away often, whereas French and Castillan/Spanish colonies almost never do. I don't think I have ever seen a Spanish or French colony break away.
The only exception here seems to be Portugal, who is a weak land power but their colonies hardly ever seem to break away in my experience. I think this has to do with the number of colonial nations Portugal has; not all of them are rebellious and so they add to Portugal's military power when the AI thinks about breaking free. Often you will see three of Portugal's colonies fight the other three in an independence war. However, if they do win then there is half of Portugal's colonies gone right away, most likely followed by the other half.
Also I think a major factor is the lack of AIs available to support independence. Only the AIs listed above create colonial nations as far as I can see. AI native nations in North and South America are useless as allies, since most of them, especially in North America, never get above 1 or 2 provinces anyways. So that leaves on;y the colonizers in Europe and rebellious AI colonies from the same root nation to support independence, plus the player. Not a big list and by the time colonial nations are looking for independance, the AIs that have them are usually pretty strong.
Plus, what do the AI nations gain from supporting independence for a colonial nation? Or the player for that matter? Sure, you get a free nation that is now vulnerable to subsequent attack in order to absorb it into your own colonial nation, but that's not really a huge benefit at all by the time colonial independence becomes an issue.
