I’ve read only a couple dozen haibun. Contemporary Haibun Online (CHO) gives criteria and examples and week 23 of One deep breath gives the challenge to write one.
CHO says it should be direct, concrete, have economical imagery, be infused with life and energy, lighthanded, playful with unexpected turns showing how the haiku came to be, such as Adelaide Shaw does.
Paris Parfait in her haibun of immediacy and concreteness, also afterwards quotes Bruce Ross, “If a haiku is an insight into a moment of experience, a haibun is the story or narrative of how one came to have that experience.”