If we distinguish between bad dialogue and gOod dialogue, then, yes, bad dialogue is indeed hazardous to ecumenism. But good dialogue, by contrast, is the very soul of ecumenism. In what follows I would like to go so far as to propose that good dialogue may even be, like Word and Sacraments, a "mark of the Church," what the tradition calls a nota ecclesiae. Goodness knows, there is still plenty of bad dialogue around, often palmed off as good, even by professional ecumenists and theologians. But there are also signs of good dialogue, maybe not overwhelming signs, but frequent enough and recent enough to be promising. So recent are the instances I have in mind that I'm tempted to refer to bad dialogue as the "old" way of dialoguing and good dialogue as a "new" way of dialoguing.
Bertram, Robert W., "Is Dialogue Hazardous to Ecumenism?" (1992). Institute of Liturgical Studies Occasional Papers. Paper 60.