This is what scrum (done well) helps with. Unless the software was horribly written, it’s usually easy to estimate effort for very modest, well-defined changes.

Take a large enhancement or overhaul, however, and estimates will be wildly off and it’s because of the iceberg effect of unknowns.

What scrum forces the inexperienced teams to do is start breaking down, plan and incrementally chip away at a ill-defined, large request by turning it into many smaller and better understood deliverables.

It’s what very good teams do, though they do it without the need for formality.