To add to this, in over 17 states there are also city or county level income taxes[1] that you have to be set up to handle.

Then there's the potential for each employee's address to be considered a nexus, since each of their homes becomes a de-facto company presence. Potentially subject to local registration, licensing and taxation by local municipal authorities, in addition to just the state level requirements. If you have a presence in the state, and your employee's official work location is that location, then you only have to deal with that local registration once wherever the presence is located. But if you don't have a location the state, or you have employees who are officially designated as work from home (rather than unofficially working from home while being designated as assigned to an office), then this quickly scale into a giant mess of regulation to navigate[2].

[1] https://taxfoundation.org/local-income-taxes-city-and-county...

[2] https://smallbiztrends.com/2018/04/remote-employee-complianc...

Why so? I though all payroll/tax details are outsourced to the accounting firms and very straightforward in fact.

The payroll taxes can likely be taken care of easily by your payroll provider/accountancy firm, but there's potentially local business licensing requirements[1] you become subject to as well. If you have an office, you generally do all of that based on the location of the office, and the employee's home address doesn't come into play. But if the employee is officially a work from home employee, some states/municipalities treat that as a presence subject to business licensing or permitting requirements with the local authorities, as well as potentially triggering sales tax collection requirements that have to be accounted for within your sales/invoicing processes.

[1] https://smallbiztrends.com/2018/04/remote-employee-complianc...

My understanding from people who have done it is that:

-- Yes, you can outsource most of the payroll-related headaches to someone like ADP but

-- There are still a lot of other state by state details that can't just be handed off to a third party and told "Make it so." It's not a particularly big deal for a larger company, but it can be a fair bit of overhead for a small employer adding a new remote employee in a state with lots of paperwork.