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> 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.

Simply reject any piece of input, such as a user display name, which uses characters from two or more different scripts, or purely typographical devices such as ligatures.

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Not quite.

Reject any mixed-script confusable (and for certain types of inputs, like email addresses, validate sub-components separately -- someone might get to choose their local-part but not their domain).

For strings which aren't mixed-script confusable, if you need to use them as identifiers and perform comparisons with them, normalize to NFKC and case fold. Doing so will eliminate any ligatures, stylistic variants, composed versus decomposed forms, or other things that people use to call Unicode "weird".

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