Even in languages that allow for them, Unicode chars in identifiers are rarely used. They carry some costs that may matter a great deal: they can make certain tools unusable with a given codebase, they can act as hurdles to contributors, and they can act as hurdles for users of libraries ("what the hell is ε?!", etc). However, if you know your audience, and know your contributors, and generally know what you're doing, have at it.
All those same costs apply if you're using Unicode characters as part of your language's syntax, and the user confusion issue is exacerbated if the characters are entirely equivalents to other bits of syntax that approximate what the Unicode chars may encode precisely.
For example: at least some parts of Akka switched over to using Unicode arrows instead of the ASCII approximations[1]. Will contributions to Akka that use ASCII arrows now be rejected? How many times will Akka leads have to point surprised potential new contributors to whatever part of the Scala spec allows for Unicode arrows? God help the poor soul that would like to do some Akka hacking on a Windows box. Etc.
All this is worth the slight prettification? Presumably, the answer is "yes". As I said, we can all make our own judgments.
[1] https://github.com/jboner/akka/commit/554bb3eb293fe083dcc4fd...