Spreadsheets can end up a tangled mess, and are more prone to errors.
All software, ready made accounts software and spreadsheets alike are all subject to the GIGO rule. If the person does not have a basic understanding of what is supposed to go where and why, errors will occur.
I do find, especially for small businesses, that spreadsheets work very well once set up. I have a number of templates which I have provided to cllients which they complete and I find it works well. It's easier for them and I get the records in a manner which makes it faster for me to complete, we both benefit.
Try writing a PRSI macro for example.
For payroll I would recommend simple software as the computations for PRSI in particular is not easy to fully cover in spreadsheets. As far as PRSI self-employed is concerned, you can do the computations on ROS.