The honest limitation: maintenance catches wear before it becomes a failure, but it doesn't reverse fatigue that's already set in. If a spring is near the end of its cycle life, we'll tell you it needs replacing rather than just lubricating it and calling it done - a spring that's visibly stretched or has flat-spotted coils is a repair job, not a maintenance item.
The other thing to watch for on 1970s-and-later sectional doors is track separation from minor settling. We check for it, but if a slab has shifted enough to throw the track out of square, that's a bigger fix than a maintenance visit covers, and we'll scope it separately rather than force a quick adjustment that won't hold.