Phillips Brooks observes that a series of sermons constrains a preacher and that the challenge is to maintain a consistent vitality from beginning to end.
Some evangelical churches that do not follow the lectionary use sequential expositions or series.
While sequential expositions have the advantage of unfolding Scripture to the congregation week by week as laid out in the Bible, topical series can be a challenge to start, continue, and conclude in a consistent way. Their length can sometimes mean arbitrary inclusions.
It calls for a lot of discipline.
I’ve been using one sort of topical series each year, mostly for the variation and interest that nine or ten weeks of focus can allow.
The only serious danger about a course of sermons is, that where the serpent grows too long it is difficult to have the vitality distributed through all its length, and even to his last extremity. Too many courses of sermons start with a very vital head, that draws behind it by and by a very lifeless tail. The head springs and the tail crawls, and so the beast makes no graceful progress. I think that a set and formally announced course of sermons very seldom preserves both its symmetry and its interest. The system of long courses is apt to secure proportion at too great an expense of spontaneity.
Phillips Brooks, The Joy Of Preaching, Kregel Classics, 1989, pg. 120.