There was some opposition including from Senator Debbie Stabenow (D-Mich.) who authored this opinion in Politico. I encourage you to read Stabenows entire piece, it's excellent.
Smithfield may only be the beginning. Experts testified at a recent Senate Agriculture Committee hearing that China is watching this deal and waiting in the wings to purchase more U.S. food companies… To be sure, the purchase of one American food company does not jeopardize America’s food independence. But Smithfield is our largest pork producer — will China or other countries seek to purchase our largest poultry, or dairy, or corn producers next? Is it in America’s security interests if in a decade or two our food supply is 30, or 60, or 90 percent foreign owned?
This purchase by the Chinese is for export back home to satisfy their own pork addiction. So the U.S. gets the pig pollution, which can be extreme, increases it's greenhouse gas emissions and the Chinese get the pork.