Without good validation (possibly including locked sheets), people will make all fashions of inconsistent entries that “look right enough” but will stymie automated ingestion.

We had some hellish “use Excel as an interface to ‘real SAP’ process” for budgeting/headcount planning and it was almost utility-free for us as users. Even doing copy/paste would break parts of it, but invisibly to the user in real-time but would fail on import days/weeks later.

You never did this, so you dont know that even the "simplest" thing like writing an address can be done in 50 differenr ways, and then someone has to clean up the data before importing.

Of course you can partially prepare for it by making a file where users have to pick things like town name from a scroll down list (hope your list has every town), but they still can get "creative" and mess it up.

It's not so much complexity as it is removing manual process steps and reducing all the risks and overhead associated with taking data on one format and loading it in to another - at scale