Fr. de Mello (May he rest in peace.) is an easy case: Cardinal Ratzinger's [now Pope Benedict XVI] department: the Congregation for the Doctrine of
the Faith, published an official warning about a year ago about doctrinal
ambiguities in de Mello's writings.
You can find the text of the statement on the Catholic Culture web site.
Meister Eckhart is described in the Catholic Encyclopedia article online.
According to that article, he wrote in an un-technical way which made his
writings prone to misunderstanding, but often corrected the misleading
parts latter on in the same sermon or treatise.
When accused in 1327 of teaching dangerous doctrine, Eckhart admitted
that some of his statements could be taken in an unorthodox manner. He
explicitly rejected those unorthodox ideas and submitted himself to the
Pope. In 1329, Pope John XXII condemned 17 statements from Eckhart's writings
as heretical, and some other statements as suspected of heresy.
Thomas Merton's writings are a mixed lot: you'd be safe reading his early
books such as
The Seven Storey Mountain or The Sign of Jonas. Catholic
Culture has a
helpful article about Merton.