I've had zero issues loading .doc and .docx files in Writer, and the same goes for .xls and .xlsx files in Calc. The only thing that might have issues is anything with VBS built-in, but that's not commonly used and I haven't run into that yet.

I will say that I agree with you on Excel's UX, but with the caveat that spreadsheet programs in general are just a pain (still better than trying to do anything manually in QuickBooks Online, though).