[chrismorgan]

I expect the number of tickets induced by supporting Unicode to be a negligible fraction. (I offer no support for this expectation.) Depending on the type of service you’re running, you might even avoid a few tickets from people that might complain about the restrictions, though certainly most of the time people will just sigh and choose something else and not even think of contacting to complain.

No security drawback? You make it harder for people to use the password they want to use. There is a real cost to that, encouraging bad password hygiene. Provided you support at least printable ASCII it’s unlikely to be a significant cost, but it is a cost, and I remain entirely unconvinced of the practical benefits of the restriction.