Windows users can access your solution if they have Filemaker for windows installed. Some interface elements will render a bit different, but you can adjust the solution to fit their needs.
FM is cross platform compatible, as siplus has stated some script steps and other element may render differently. Another great reason to use FM.
please see this link:
You have the HOST, you need the client(s).
"A copy of FileMaker running on Mac OS or Windows, a copy of FileMaker Go on iOS, or a web browser can act as a client to connect to a hosted FileMaker file."
If you go the web way you might have to purchase some additional concurrent connections.