Spreadsheets are fine. I also use a database along with a mailmerge facility to combine invoices for commercial work. The database is also set to prnt out my work sheets if I need them. It works just fine. If I had a larger business, I would do it differently though. It was difficult to set up and I did it long before I was aware of other software - or even CIU for that matter. Although it can seem clunky to an outsider, it evolved over time. It didn't just set it up that way and then leave it. Also, it gives me a degree of flexibility as I can change things myself rather than waiting for the software writer.
I suppose it depends what you're used to really. I've been using a spreadsheet for this way back into the early 90s. Back then I had an Amstrad PCW with a memory expansion pack in the back and no hard drive. Although the machine was basically a word processing machine with a couple of extras, there was a spreadsheet called "Rocket" (what a laugh!) that you could buy for it. The program was on an old 3.5" floppy. I held the data on another floppy. I could only fit 6 months worth of work on a disc so needed two discs for the whole year. The expenditure list required an additional disc. It could be slow because, even with an extra memory pack, there was insufficient room to store it all in memory, so it had to regularly access the disc. A file could easily take a minute or so to load.. That's why it's no hardship for me to dispense with the pre-written software available these days