I've been tasked to rewrite a spreadsheet as a dedicated app.

Most of what is in TFA is correct but it only begins to describe the problem.

My takeaway is very simple: the world runs on broken spreadsheets that are full of bugs and wrong assumptions. And the PowerPoint presentations are shown to business people taking decisions based on the wrong numbers coming from those spreadsheets. And it's the world we live in.

> It was common for the data to be split over two or more sheets.

And when the sheet are separated, for example, by years, that's even more bugs. Stuff is counted twice. Others not at all. The concept of when an entry should be "closed" when it's opened on year X and closed on year "X + 1" is something that bewilders spreadsheets users.

> and errors as the spreadsheets grew or were adapted to new data

Errors in spreadsheets are the big one: spreadsheets are full of errors.

> Another problem with spreadsheets is that the amount of data they can contain is more limited than when you use other data analysis workflows.

Another gigantic issue is that the notion of time is broken in dynamic spreadsheets: not because a spreadsheet cannot be written to correctly deal with it. But because the spreadsheet users don't know how to properly model how values relates to varying time (typically the spreadsheet shall work, for the cell that fetches the value, for the time value of "now" and that's it).

> I know there are lots of people who love using spreadsheets.

It's because you can cosplay being an actual programmer when you're not. The result, sadly, is exactly what you'd expect: buggy spreadsheets full of broken assumptions.

Which are then sent to those meaning real business to be rewritten as dedicated apps...

P.S: as of now I'm working on finance stuff... Same old story: a spreadsheet that has overstayed its welcome, it became gigantic. It's a pure mess of fetching values (and fetching way too many values, which creates technical issues) and bogus little things left and right. So what are we doing, again? Porting that spreadsheet to a proper dedicated app that can, correctly, deal with a proper amount of data, while fixing all the little glitches and gotchas too complicated to fix in a spreadsheet.