In my experience Unity handles most o the work building between PC platforms but you will want to ensure some stuff like inputs work correctly, along with in some cases the way you set resolutions/cursor locking and even some graphical effects will look a bit different.
I would not suggest ever officially supporting a platform you can't test yourself because there are too many "gotcha's", but if you know someone with a mac or linux you should be able to sort out a working version pretty quick. And yeah most modern linux builds will be fine with Unity, though if you rely on shadows there is a known bug where Intel HD GPU's have shadows forcibly disabled by Unity on Linux (It's a performance thing from a few generations back that's unnecessary now, so hopefully goes away soon).
Edit cos I'm at the top of a new page here's the banner artwork we made for our PAX Aus showcase booth (Yay for free booth
I'll remove this if it's too advertising-y but I'm really proud of how it turned out, we've been trying for a while to find the right style for our promo art)
Double edit - Oh one additional thing with Linux is I don't think Unity sets up the executable's properly by default (because you're building to windows and it sort of can't), Tim handles this step so I'm not 100% sure what the exact scenario is but look in to making sure the executable is set up properly if you're having trouble running a build on Linux.