problem is that it does too much out of the box. Finding some sloppy formatting - ok. Fixing it automatically? even better. But rubocop tries to sniff "code smells", security problems, and it's not that great with it, not everything you can get about the logic and intent just from AST. There are so many rules that developers spent significant time appeasing rubocop and this sometimes takes away effort to make code actually readable by humans. Have you ever seen a tidy mess?

Even worse, it gives simple tools to write plugins to sniff more logic related stuff, and I've seen projects where developers really went overboard with it.