Runes of Magic actually ran pretty well in Linux/Unix. I was playing semi-regularly on Fedora 20 back in the day and it was noticeably more stable than Windows. There were a few WINE addons and programs you had to install and some interesting settings but it worked really well.
However, when the item shop went to browser based, that's when the usability went out the window. The item shop was/is completely non-functional on *nix. Therefore, it was deemed "non-playable" due to the dependance on the item shop.
Best stick with Windows. If you don't want to buy Windows 10 you don't exactly have to. As of yet an unlicensed copy of Win10 is nearly 100% functional, just a taskbar notification and inability to change the desktop backgrounds or personalization are all that's missing.