The problem with debugging formula code is that it's hidden in the default view, and you have to know which cells to click on to find each bit of code.
In simple spreadsheets, the proper cell to click on is obvious, because the formula only updates the one cell where the formula is. The problem comes with multi-cell functions like Excel's VLOOKUP or Google Sheets' QUERY, where a cell that looks like it contains literal text might contain the complex formula call, and then all of those around it might look like plain text, but they're also the call's results. Editing any of this wrecks the sheet.
Despite these dangers, I recently perpetrated such a thing in Google Sheets, because that's the only platform I could get to easily work across many sites, some of which have users that are often offline. The next step up from that would be to stand up a custom web app, and the project simply did not demand that amount of effort. The next-best idea was to keep passing around .xls files via email; barf.
That's the seductive nature of these one-off spreadsheet "apps": they're quick and easy to stand up, yet difficult to extend and debug by the time that they grow to the point where they would justify development time for a proper application. By then, they've also gained a big enough user base and feature set that rebuilding the app properly also means a big effort in all axes: development, testing, deployment, and retraining.