Look, his writing wasn't exactly Harold and the Purple Crayon. But then again he wasn't writing for four-year-olds either. Nevertheless, folks' main beef with our main man was that he wrote, um, incomprehensibly, to put it mildly.
In all fairness, Lacan's writing can be a mind bender. He uses a lot of jargon and tends to go on and on and on (and on some more). This tendency made him a huge target for criticism. Not only was his writing called purposely obscure (a "tissue of horrors," according to one critic), but he was also accused of abusing scientific concepts and creating an "incoherent system of pseudo-scientific gibberish" (source).
According to one fellow, his bad science caused more harm than good for his patients. Another agreed with this assessment, asserting, "His lunatic legacy also lives on in places remote from those in which he damaged his patients, colleagues, mistresses, wives, children, publishers, editors, and opponents" (source).
Some critics said he fashioned a cult, and two scholars even wrote a book called Fashionable Nonsense (that's gotta hurt!). In one review, Lacan is called "The shrink from hell" and labeled him an attractive psychopath who exploited his good looks "in support of his boundless appetite for wealth, fame and sex" (source). We could go on, but we think you get the point.