When one imports a CSV into Excel, Excel has a nasty tendency to actually change the data based on formatting rules. When you open up a CSV file in Excel, it silently corrupts the information. You need to back up your file before doing any work on it.
Excel refuses to do proper Unicode, or at least I haven't found the right way to reliably get a Unicode CSV out of it that won't break in third-party tools.
Excel does not do versioning natively, and you have to rely on a buggy mishmash of Microsoft-based solutions to do file-based version management. Excel files tend to get emailed around everywhere, and so there's never a single source of data. It has the worst properties of a decentralized architecture without any of the benefits.
Data belongs in a relational database. You can use a spreadsheet to manipulate it, but the manipulated data needs to go back into the database. The spreadsheets that do this are programs and they need to be managed as code, going into version control systems.