The thing is, May is in a position to do untold damage to the future of this country by dint of when she is PM, at a critical point in UK legislative history that will change the way we deal with the world for decades. (If she were just a bit useless during a time we wren't ripping the country to sheds, she'd be much less worrying)
Trump is able to do untold damage to the US and the US reputation abroad simply by dint of being Donald Trump.
I shudder to think of the carnage that could be enabled by putting Donald Trump in charge of a seminal moment in UK history. It's a double whammy.