He wasn't a scientist, his understanding of basic physics was very weak, and he didn't rely on scientific methods to arrive at his conclusions, most of which were ludicrous. His defenders will say "but he got this right, or that right," while ignoring that the vast preponderance of his conclusions were faulty. He might have been right occasionally, just as a stopped clock is right twice a day.
I encourage you to do a web search on debunking Velikovsky; there's a fair amount of interesting material that deals with his mistakes.