How hard was it to maintain that nginx only Windows server compared to doing the same on Linux?

Not much. Standard nginx commands work in cmd (like "nginx -t"), and most of the work I did was kicking out one of the production servers when new deployment was in order and letting it back in to serve requests after it finished.

On the bad side, I just couldn't be bothered to create a script that would do that automatically, which resulted in one or two downtimes when I accidentally just tested the changes instead of reloading nginx for them to take affect ("nginx -s reload"). Repeated boring work lead to more carelessness, which would absolutely not happen if it was on any Linux system. I'd create an Ansible script(s) instead and wouldn't have to log into the servers manually for that one frequent use case.

One thing I wonder though, is that whether Windows has monitoring tools like Monit, so that incidents are automatically detected and reported.