Everything you can do in NarraFirma you can do in a spreadsheet. It just takes longer. On the other hand, a spreadsheet is much more flexible. As software designed specifically to support participatory narrative inquiry, NarraFirma has certain built-in assumptions about what you want to do with it. So does a spreadsheet, of course, but the assumptions behind the design of spreadsheets are very broad, so you can use them more flexibly.
If you look at NarraFirma, and it looks like it will help you do what you want to do, start by using it on a small project and see how it works for you. If you look at NarraFirma and it does not seem to match what you want to do, a spreadsheet might be better.
Or you can use both. I often supplement my use of NarraFirma with the judicious use of spreadsheets for custom comparisons and graphs, as I need them.