Swedish NIs are insanely strong to start with, they have wonderful production, huge number of ports to build naval limits up and can get several nice friends to support their independence. If Denmark got no strong allies and Sweden gets a bit of time to build some navies, war is tilted into Swedish favor. If Denmark allies with Poland who PU-d Lithuania, Sweden is pretty much a history. Danish NIs aren't much for a conquest game, but they are very good for trade and colonization. Problem is that AI doesn't utilize it and focus on bizarre expansion into poor Estonia or overly complicated Northern Germany, too strong to deal with early on due to HRE.
Although people complain about poor Denmark, Sweden vs. Denmark/Norway is an even fight, unless one of the sides manages to acquire a good shock general. With 1.13 strong Denmark is far more usual sight than ever before.