Even if we assume that GMOs have no downsides and they solve world hunger. The problem is actually far simpler than you might think: Solving world hunger is impossible by increasing supply since demand will rise accordingly. The only solution is to decrease demand especially for meat since most plants are fed to animals. If we didn't "refine" our plants into meat then we could have instead fed the entire world population already without GMO. World hunger is largely a political problem and often it's solved by simply freeing up the markets and allowing global trade.

The disadvantages include things such as vendor lock-in into expensive proprietary seeds that you can only obtain from a single supplier with compulsory proprietary herbicide/pesticide and you might become a target of lawsuits because cross pollination cannot be prevented.