How adequate a curriculum is perceived to be depends upon the perspective of the person assessing it. A department, or school, or school district without a curriculum is handicapped in its pursuit of institutional support. Leveling not infrequency reflects some theory of learning development. The curriculum is thus, basically, an administrative aid for the teacher, rather than a pedagogical one. Any teacher who has ever felt rushed to finish a unit to meet time or testing requirements knows that pedagogical considerations alone do not determine the curriculum.
Inexperienced teachers look to the curriculum as a cookbook.
But the rhythm of classroom interaction cannot be anticipated in a document never intended to anticipate that rhythm. A curriculum is a compromise of teacherly, administrative and public interests. A curriculum adequate for administrative purposes may still be perceived to be inadequate for teaching purposes, e.g. pacing, coordinating classroom activities with texts, etc. It all depends what you're going to use it for. This is a misperception.