Depends on what you randomize. In a Link to the Past I don't think you can, because most of that game is execution based. It maybe unbeatable to someone like us, but not someone really good at the game.
In Free Enterprise you can get randomization that are unbeatable. Like getting only Edward and Tellah and having to beat boss in a hard location, but that really only happens if you choose the flags with a red box (which means it makes it difficult.) However, for most normal flag sets I would say almost all of them are beatable.