It doesn't really need to be all that specialized --- for example, US payroll. Every business with employees needs this. The few FOSS options that exist are mostly focused on countries outside the USA where things are much simpler.
US payroll is complicated by the fact there are 50 different state tax authorities and a multitude of private industry deductions and requirements --- 401K contributions, court ordered garnishments, time off accruals, etc.,etc.. Plus keep in mind that all this is subject to change on an on-going basis and failure to keep up with the changes can incur significant penalties and interest.
Once you start looking into it, you come to realize and appreciate how/why business is unlikely to ever be fully served by FOSS. Proprietary/for profit software will always play a significant role.