New York City, as an example, has something like 15,000 variations for payroll categories. Plus garnishments, domestic relations orders, etc.
How do you compute the extra boot benefit for a fireman who has worked more than 526 hours in a 13 month period?
Or how to you compute the salary of a garbageman who is paid by the ton of garbage hauled, instead of the hour. Except for when they haul ashes, in which case the tonnage is adjusted, or on Sundays, where they get overtime as well. What are the business rules for an ash hauling garbage guy earning overtime on Sunday, which is also a holiday?
The NYC consolidated payroll system, "CityTime"... has cost over $700M since 1996 to build. (That included some serious fraud.) Fraud aside, Accenture was raping & pillaging too -- charging $400/hr for college grad coders.