Trust contests are very similar to will contests with similar allegations of incompetency and/or undue influence. The difference is that there may be only a very short time to decide if you are going to file a contest.
There is a procedure in the Probate Code (§16061.7) where the trustee gives notice to all beneficiaries that the trust has become irrevocable and a copy of the trust is provided; if that procedure is properly followed, then a contestant has only 4 months in which to bring a trust contest.
If the contestant waits too long, or there is a no contest clause, then they may be out of luck. That’s not to say that if you can prove the entire trust is invalid (and therefore the Probate Code §16061.7 notice is invalid) you can’t win; you might. It just makes a hard job harder.