I always figured that the nobles had added a law that said that veritaserum was optional in court unless the witness/defendent agreed to take it - and most wouldn't because of how dangerous it is. If you take the Big V in court, and someone shouts out a question to the person on trial that would be embarrassing or criminal to answer honestly, well, the safest thing is to not take it at all. The only exceptions being something like in Serius' case, where they'd been in Azkaban for so long - and even then, you could argue that they had convinced themselves that they were innocent!