There's nothing whatsoever wrong with a shared economic area and huge economic disparity. Consider Bombay, Bangalore or Pune - all lovely places which bring high skill/high income folks together with low skill/low income folks looking to move up in the world. The result is beautiful modern neighborhoods like Koramangala, Bandra or Koregaon park, up and coming areas (described by westerners as "slums") like Dharavi or Baiganwadi, and free trade that benefits everyone. You have rich Parsis/Gujus moving in and working at banks, smart Tamils working in software, poor Biharis driving their Ubers, and everyone benefits.
Do you think Bombay should close her borders to Bihar, or even inland Maharashtra?
The only big issues are political externalities - i.e., Mexicans might vote for politicians that make the US more like Mexico. That's an issue even in the US (California refugees voting for CA-like policies in their new states), and is best solved with disenfranchisement/reduced democracy rather than trade barriers.