It's a similar trade-off to using Terraform to manage a resource, compared to just modifying some cloud resource directly.

You pay a higher upfront cost, but you end up with declarative code which has advantages for maintenance.

With NixOS, rollbacks from changes to system state are trivial; and the whole system is configured from a single starting point, which makes it easy to track down why things are a certain way.

For a personal workstation, it's not necessarily a practical trade-off.