OS matters way way less than it used to (Think of the bad old days, where Macs couldn't read Windows floppies), but it can still be a pain. For example, Google Music integrates best with Android. Microsoft OneDrive is most slick on Windows. iTunes runs like crap on anything other than OSX.
You can get away with any hodgepodge mix these days if you want, but it's still more convenient to match.
>> iTunes runs like crap on anything other than OSX.
I know a lot of Mac users who would beg to differ.
I use Dropbox, Google Drive and OneDrive across all my devices, and I don't have many complaints at all. YMMV, of course. My use case is probably much simpler than yours.
It is still quite awful, especially if you have a large media collection.
That said, one benefit to a mostly homogenous ecosystem around apple devices, at least for me, airplay and streaming media across devices with less work (I get lazy about home it issues)
You can get away with any hodgepodge mix these days if you want, but it's still more convenient to match.