That it has a lot of my old code working perfectly for years, and I don't have spare time to port it to another language and keep catching subtle bugs for months.

Out of curiosity, how many lines of code are we talking about here?

"Port it to another language". You're saying this as if the minor asinine breaks in python 2 Vs 3 make it a whole new programming language, when in reality it is mostly bullshit like print going from statement to function.

My problem is some libraries which never transitioned from 2 to 3. I don't want to try to convert them, particularly as they involve Unicode which is one of the hardest bits to get right in my experience.

Improved unicode (string vs byte separation) handling was exactly one of the larger reasons for the breaking change to 3.

You are right that unicode is much better in 3, but in my experience this is a pain point mapping programs 2 -> 3, because often programs seem to work fine until there is some exciting unicode, or malformed characters, in the input, then they crash when they used to just rumble on.

That sounds about right. Fail fast and loud so things have to be fixed, rather than continue silently and likely corrupt data.