I don't like storing my boat outside period. In the summer I don't have anywhere to put it indoors though. Sun is hard on stuff, I hate tree goo, bird poo. If I had the money it would sit in air conditioning. Freezing water as TEAMVMG says is the major issue. Getting all of the water out of everywhere is borderline impossible. All it takes is a few drops in the wrong spot expanding when frozen and blowing stuff apart to create issues. Last winter I knocked my boat apart and slid it though a window into the basement. The year before I stuffed it into a unheated shed at the in-laws and the cats pissed all over it.

I don't know where you are in Wisconsin, but winter is F-ing long up here.