I've used both for a long time (relatively so for Sheets given its age). The online aspect aside, Sheets does tend to work for 90% of my use cases. But that last 10%? It doesn't come close to what I can accomplish with Excel. Exotic dimensions in data, frankly better pivot table support, and perhaps most maddening, the much faster macros....
I created a simple function/macro once in Sheets to help me indicate when a group of rows were done; I had to watch my 3-5k sheet painfully iterate through every line and assign new colors. The script system even gave up at some point after I closed my laptop(? So much for online!..), and I had to manually restart it.
