Two comments have come in from experienced practitioners on the question of depth vs breadth in church planting movements.
Both of them are worth repeating.
Almost always the argument about a mile wide an inch deep depends on the argument that new Christians are not deep
What they always means is that they do not know enough. Not enough formal schooling, not enough degrees, not enough knowledge of doctrine and history of their own tradition.
Depth in most seminary educated leaders is equated with accumulating knowledge. Discipleship in the NT defines depth as how obedient you are to what you know, not how much you know
The Thessalonians had three weeks of formal teaching yet Paul praised both for their deep faith and their ministry.
Though the gospel spreads quickly, it isn't superficial. Each household comes to faith through intense discipleship to obey all the commands of Jesus combined with a passionate fidelity to the Bible as the true word of God.
In my own experience, three months to four months of discipleship is about how long it takes us to go from first discovery Bible study to baptisms. About five months later this family has reached that stage with another, while we have also reached that stage with yet another family as well. So the speed is not in a superficial evangelism, but rather the exponential growth of harvest workers.
I'm beginning to think depth vs breadth is a false dichotomy. Maybe you can have your cake and eat it too!