Having to relearn the whole stack. Networking, virtualisation, UI of the OS, web servers, database, scripting, programming language and IDE compatible with the OS, and for that programming language desktop framework, web framework, finding and understanding 3rd party libraries.

None of this is my day job, it is knowledge and muscle memory I accumulated over 20 years being invested in and toying with the microsoft ecosystem (.net, visual studio, sql server, iis, hyper-v, etc). Changing OS means pretty much restarting from scratch. It is tempting (I am deeply uncomfortable with the new Microsoft) but I just can’t afford to invest that much time and energy.