Compared to any other enterprise app, absolutely! The easiest, by far.I don't know if I'd go that far, but of the 30 or so subsystems we sell our customers, I'd put it in the easiest 5 or so.
Here are some factors that make it easy:
1) The underlying domain model is well understood and has been for decades. Employee works for you. You pay them wages and benefits, set aside taxes, and mark that you did so. Periodically you export your records to show people. Changes in the overall procedure practically never happen. Changes at the data level happen all the freaking time but, hey, computers tolerate data changing pretty well.
2) The users of the system are all experts. They should be, using the payroll system is their job. They have a high tolerance for complicated workflows and will never push a button just to see what it does.
3) Performance demands on a payroll system would be rounding error on any other system in the enterprise. Even with 60k employees you have, hmm, a few dozen payroll clerks, if that? Changes take weeks to propagate anyhow, so if a report isn't quite up-to-the-nanosecond you don't have to sweat it.
4) I am not speaking about my employers or my employers' customers: many enterprises do not want fantastic payroll systems. If it has a little black magic involved and some sharp edges, that suits the payroll clerks perfectly fine. After all, that means there will always need to be payroll clerks who know about those little usage niggles.