It depends on what they did.
I won't go into specifics, but I've known a number of lawyers who were trained by an attorney with the worst reputation in Canada and they have all turned out to be fantastic. In fact, in the city where that lawyer works, it's commonly known that if you can survive that firm, you're great and a lot of people line up to hire lawyers who went through their program.
Yeah, this isn't going to help, really, but there are disciplinary issues and there are disciplinary issues. To use a non-legal example, Dr. Henry Morgentaler would be rightly described as having a history of disciplinary issues also. Some people really do get in trouble because they are pushing boundaries, advocating fiercely, etc. That said, most of the people who get in trouble are simply failing in administrative ways - they simply aren't organized enough to do all the critical things they are supposed to be doing. I'd encourage anyone to look closely and skeptically at any potential employer who has disciplinary issues, but not write them off entirely.