Huey Long was called one of the 'most dangerous men in America' by FDR and he inspired Sinclair Lewis's "It Can't Happen Here"- what was it about him that caused such worries? Was there any substance to their concerns?

In case you don't see it, I gave a more through response to a follow up question, but in short FDR believed that a failure of the New Deal would lead to revolution, and he pinpointed two particular figures as particularly dangerous: either Douglas MacArthur who might lead a conservative backlash or Huey Long who might lead a communist uprising. There was some reason to worry on that front: although the modern American left often points to the New Deal as an example of the possibility of their politics, at the time it came under quite harsh criticism by prominent people on the left for not going far enough, and none were more prominent than Huey Long, who could point to successes as governor and was the head of the popular "Share our Wealth" society. Long also possibly had some personal antipathy for Roosevelt and felt he was passed over for a Cabinet posting.

"Huey Long and the Presidential Election of 1936" by Robert Snyder covers your question pretty well.