As soon as multiple people try to use/modify the sheet, you end up with "Bob added a handy equation to track X, but Fred just wants to change Y but that breaks Bobs equation".
Sheets are infinitely versatile, but because they don't enforce any kind of abstraction, you can quickly make yourself a spaghetti of equations/macros.
I realize this is an unrealistic proposition, but think of all the principles we've developed (as an industry) that spreadsheets violate - no separation of code and data, prone to repetition etc.
https://www.youtube.com/watch?v=FZi9CSB9_kk There was a talk at DConf about this with a bit more depth than what I've written here, and potential replacements
You can get around this be having someone engineer a spreadsheet using all of the advanced functionality and VBA that Excel offers, but by that point it becomes a lot easier to just use an application that's built for whatever purpose you're trying to achieve.
Spreadsheets are accessible for a lot of people, but that doesn't mean they're easy to use and maintain. "ease of use" implies "use" for some purpose.
A lot of time ease of accessibility (anyone can click an icon and start entering numbers and doing calcs) conflicts with actually achieving whatever your purpose is. And if often conflicts with maintainability.
Spreadsheets ease of accessibility is both their biggest benefit and their biggest drawback.