<p>Yeah, except for the tremendously strong problem that a lot of people in Shanghai will prefer to speak Shanghainese which is completely different from and mutually unintelligible with Mandarin. Plenty of people in Shanghai are conversant in, but are not completely fluent in Mandarin, and some people in Shanghai (particularly older people) can understand Mandarin, but are not able to respond back to you in Mandarin with anything more than basic phrases. </p>
<p>The point of going to China to learn Mandarin is to completely immerse yourself in a pure Mandarin-speaking environment. You will not get that in Shanghai or most Southern Chinese cities. Instead, you will find yourself immersed in the dialect of that city. That's not going to help you learn Mandarin.</p>