It's controversial merely because he said biggest. I l, and most likely many others, would argue that the bigger problem is the fact that we have introduced so much greenhouse gasses into the atmosphere that life as we are now will be impossible a hundred years from now
I think the bigger issue is how to maximize individual freedom while preventing school shootings, dictators and potential oppressors from taking control of systems
I'm not sure if I had to pick one issue what that issue would be. Probably either pending climate disaster or the increasing centralization of wealth and power.