Jean Bodin answered Weyer in 1580 with an argument that tells you what the risk was. A man who defends witches this warmly, Bodin suggested, is not merely mistaken; his sympathy is itself evidence about him. That move, treating opposition to the trials as a symptom of complicity, was available to anyone at any time, and it is the reason so few argued in print. Weyer had a patron, the Duke of Cleves, and a position at his court. Without that protection the book would very likely not have been publishable, and its author would have been vulnerable to exactly the inference Bodin drew. It was placed on the Index of Prohibited Books.
Later readers have consistently enlarged him, and the enlargement should be resisted. Weyer did not say that there is no Devil. He did not say that magic is empty. He did not say that the whole category was a fiction, which is what Reginald Scot would argue in English twenty one years later. His case was narrow by design, and its narrowness is what gave it a chance of landing.
In the nineteenth century, psychiatry adopted him. Editions appeared presenting De Praestigiis Daemonum as an early textbook of mental illness and Weyer as a founder of the discipline. Historians of medicine treat that claim cautiously: the language of melancholy in 1563 is not the language of clinical diagnosis in 1860, and reading it as such takes the book out of the argument it was actually in. Which does not diminish what he did. He looked at a condemned woman and asked what was wrong with her.