I don't object to him as an Ecumenist, as he was not an Ecumenist. He had Modernist tendencies, though, in my opinion.
To understand Schmemann it is necessary to understand his context. He was reacting against people who rigidly reduced Orthodoxy to rules and regulations and stiffled the spirit out of Orthodoxy. People who went to confession because it made them "worthy" to commune and made them worthy to be parish members. People who believed that baptism was best done in the living room of a house on Saturday afternoon. People who didn't go to Church on Holy Saturday for the Vesperal Liturgy and expected the priest to not even serve it so that he would have time to bless their Easter baskets.
In that context then, is it any wonder that he was against formalism? He wanted to reinvigorate the spiritual life of immigrants who were in a new context and for whom spiritual life, formerly tied to village life, was being snuffed out in a new environment. He did many good things for the Church, the best of which was restoration of frequent communion.
Schmemann's books are often very good, and reflect a love of Christ. But the modernist tendancies are there, in that straw-men villains are created and knocked down as contributing to the above problems. To see this played out explicitly, one should read his Journals, which I think should have never been published. In these Journals, one sees Schmemann's overly critical, haughty, vicious criticisms of others. You see him criticize an Orthodox priest for wearing his cassock to Education Day, with Schmemann attributing to him pride and a belief that "externals save." What utter judgmentalism! Orthodox of the past are routinely dismissed as ignorant. Others are wrong, but he is right, etc etc. This attitude is the true problem with Fr Schmemann, not "ecumenism" as he was not one.