I have been on Duolingo for a year, and agree with your assessments, but only I believe it is a very powerful tool when used in combination with other tools to achieve the level of "immersion" needed to become fluent. You can use Duolingo, but you need to not only be able to explore language through many mediums, but be able to apply it socially. I don't learn completely unless I speak it with other people or watch movies and try to translate them.