Unicode identifiers can be perfectly well-defined, and many languages have them. The general idea is an identifier can start with any character that has derived property XID_Start, and the remainder can be any characters that have derived property XID_Continue.
different characters looking identical, or the same character encoded in different ways
Follow the recommendation of UAX #31 and normalize identifiers to NFKC prior to comparing them. For example, the ligature variants in the article should not -- if recommendations are followed -- be different identifiers. Here's some Python (3):
>>> import unicodedata
>>> raw = ['vpnTrafficPort', 'vpnTrafficPort', 'vpnTrafficPort', 'vpnTrafficPort']
>>> normalized = set(unicodedata.normalize('NFKC', s) for s in raw)
>>> normalized
{'vpnTrafficPort'}
The real difficulty with Unicode identifiers is in places where you really can't avoid Unicode: user inputs. Those who don't read Unicode technical reports are doomed to suffer the moment they build, say, a user-account system that comes into contact with the real world.
When you allow Unicode identifiers, you don't "internationalize" your code
No, you let other people localize their code. We (the tech world) spent decades making everyone else learn English as a prerequisite to learning a programming language. Now we have the ability to lessen that burden and let the non-ASCII world (which does in fact include English!) spend less time learning English and more time writing code. We probably should do that, even if it seems icky to you.