client:
yes, most modern browsers will support and it degrades well (socksjs if no websockets)... But there are a lot of client side dependencies you will probably use, eg: jquery, amplify, etc...
there is an extra package for IE redirect if not "nice" enough: https://atmosphere.meteor.com/package/redirect-ie-7-lower
server:
not really on windows... they have makeshift support for it now. I've had no issue on ubuntu (most recent 2 versions) nor osx (1 version old). Meteror's dependencies are mostly abstracted by NodeJS.