I'm probably underselling it actually.
Since excel 2013 came out several spreadsheets which used to function in 2010 no longer do, and anything over ~50mb has become significantly less stable.
I've spent literally days debugging excel tools with VBA split all around between sheets, modules, classes etc with no documentation. Not to mention that additional logic lives in the various formulae inside worksheets (which may or may not be dynamically updated multiple times during code execution and then trigger a worksheet_changed macro).
The value of source control is pretty limited for an excel sheet, and I've been in a situation whereby a source controlled sheet could no longer be saved without excel crashing. The end solution was to copy all of the modules/sheets/extraneous code into a new workbook. A highly manual process. It appeared to work - but good luck proving I didn't screw it up.
Links to other workbooks are significantly more unstable in 2013 (they're probably a terrible idea anyway, but the company considers it the best way to affirm the data being used for calculations is right).
Excel 2013 launches differently to 2010 breaking every excel tool which relied on launch args.
I can probably find a bunch of other reasons that excel is something that literally gives me nightmares. I'm not exaggerating, and based on a few conversations with people in various finance companies this is far from isolated to myself and my company.