The problem that arises is that the characters are not handled correctly in the client system. Anything that is pulling data or working with the interface that uses these characters is not able to read them correctly.
For instance, if you use Cyrillic in your User ID, and join a server with a server manager. Anytime you use an action that puts a call to the database has a chance of corrupting the data and crashing the entire server. (Have had this happen more than once on servers I've managed.)
For just text, sure it was fine. For anything else, it had a large chance to completely crash the entire server. Maybe support for these characters will be returned after experimental, and are just held out temporarily like the FOV change option is. I don't know, and cannot speak for the developers themselves. I can only make a guess based on my own experience.