From my experience as a programmer who did accounting for a while: Accounting is the easy part, but the borders to taxes are nonexistent (at least here in Germany). So when you do accounting you always do taxes to a certain degree. And this is not something that can be understood with logic, it’s completely arbitrary.

On software that was written by accountants: look no further than DATEV. It’s an abomination I would not inflict on my worst enemy.