While not as frequently as wills, trusts are subject to challenge for improper execution, lack of capacity, forgery, undue influence, fraud, and duress.
All too frequently lawyers and commentators say that the way to avoid a will contest is to use a trust rather than a will. While that may be technically true, in reality it only substitutes a trust contest for a will contest.
If a person lacks the capacity to execute a will, they probably lack the capacity to execute a trust. And thus the trust is invalid. Similarly, if someone has been unduly influenced to write a will, they could also be unduly influenced to write a trust.
Because wills are more common, will contests are more common.
There are some differences. In a trust contest the burden is on the contestant to show that the settlor lacked capacity. In a will contest, before the will is probated, the burden is on the proponent to show that the decedent had capacity. In a will contest the question is testamentary capacity; in a trust contest it is contractual capacity.
Ikard Ratliff is regularly involved in defending and contesting the validity of trusts.