A trustee does not need to know the law well. It is to your advantage to find somebody you know you will always get along with, and with whom it is easy to communicate. It is more important that the trustee be willing to follow your wishes without questioning them, than to have a trustee that is knowledgeable. Then that means if there is any particular type of legal knowledge you need, it would be more your responsibility to learn it. This way, you are still the one calling the shots, even though you have willingly and purposefully turned over the "legal" control of the trust to the trustees.
On the other hand, the reason we say it requires more understanding for the person who sets the trust up to be knowledgeable if he or she wants to be the trustee, is that then that person has both legal and practical control of the trust. That person should then really have a firm understanding of what he or she is doing. By playing the role of manager instead, and letting someone else be trustee, the two types of power are separated into two people – the legal control in the trustee and the practical control in the manager. Then the burden of responsibility is less on each person, and therefore the need for comprehensive trust understanding is less.