The biggest drawback of globalization is evident right here in America as we speak: Not everyone is on board yet with the idea of living with other races/cultures. Although governments may agree on free immigration and travel, the local populace could very easily fall into chaos through racial tensions and xenophobia.

Even if the populace agrees to allow in new races and cultures, there's the problem Europe is currently facing with successfully integrating and assimilating immigrants into their new homes. It takes work to resettle migrants. They need housing, jobs, and someone to teach them the local language. You can't just let them off the boat and have them fend for themselves.

Free trade and loose regulations allow multinational corporations to move to third world countries, which in turn puts local workers out of work while exploiting uneducated cheap labor in the new host countries.

Multinational terror cells such as ISIS will have access to technology that can recruit members from all across the globe, and hostile governments such as North Korea can commit cyber attacks that potentially shut down online international commerce.

There are many concepts and side-effects of globalization that need to be accounted for when pushing for globalist policies. I'm not trying to push for Nationalism or Globalism in this comment. I just think it will be something that the governments and the governed will need to put a lot of thought and resources into if they want to make it work in the future.