Well, if the data you're ingesting/parsing/etc is anywhere near well formed or having a known structural definition, you should be able to mock the code handlin

Well, if the data you're ingesting/parsing/etc is anywhere near well formed or having a known structural definition, you should be able to mock the code handling it reasonably well, even up to the point of covering many weird edge cases. And even with perfect data up front, you would still encounter bugs in your production code at some point. In fact, isn't this usually the case with any application not yet in production? As an example, I had a teammate who would get blocked on needing JSON that matched a production set of data. Yet, she knew what the various fields of the data would most likely contain. She ended up writing the code anyway without the "production" data she asked for and it worked great minus a few small changes. It was ultimately just psychological block, IMO.