Plots of a bildungsroman are, on the whole, of a much looser weave that, say, an epic fantasy. In an epic fantasy the conflict of the story is between the hero and the Dark Lord, which requires tying down of the action and events. Too many happenstances lead to muttering and the exasperated quoting of Aristotle:
The events do, however, need a certain intrinsic probability to them, which is why bildungsroman are easier when mundane. An out-of-the-blue announcement that the crotchety great-aunt is visiting is not out of the ordinary; we all know of the existence of great-aunts, and that some are crotchety. The announcement that the Golden Stag, which grants wishes, appeared in the Golden Wood needs a little more foreshadowing. And probably subtle stuff, so that it seems like part of the local color.
Because a bildungsroman -- to change the metaphor -- is like a meandering stream weaving its way through the park, while an epic fantasy is water cascading down a gorge in a thousand waterfalls. And once you have got the meandering going, you do not want to interrupt with a sharply focused, one conflict gorge -- especially not if you go back to the meandering afterward. I once read a novel where the writer couldn't make up her mind which she was writing and so jerked me back and forth between incidents in the life of the heroine and an external conflict driven story. It doesn't work like that; in a bildungsroman, the main character's development is always central. They can be mixed, but in that case, the external conflict has to be the driving force of the character development, and vice versa, so you can't interchange them.
Its one real plotting advantage is the planting of guns on the mantelpiece according to Chekhov's famous rule. Because the incidents are important as they happen to the character, you already have their first purpose, and can produce them again, later, for other purposes.
Its one real plotting disadvantage is that you still need a climax to it. A crescendo. Something to sum up the conflict and bring it to a conclusion. And since it's internal, you somehow need the setting to marshall a suitable demonstration of what he has learned. Meanders do not lend themselves to dramatic conflicts. I notice that in such works, there tends to be more focus and cause-and-effect in the incidents as the work goes on, so it does not seem disparate if it -- well, a gorge would overdo it, but often it ends in a waterfall to bring the tale to a conclusion.
*Just in case: A bilgunsroman is a novel of education, a coming-of-age story.