I *thought* I mimicked what C stdio did ~20 years ago...  I'd be happy to follow what it does today if it changed or if I made a mistake.

That said, IMO:

Line-buffering should be good enough since in practice errors messages are always terminated by a newline.

I'm hesitant to make it line-buffered by default when directed to a file, since this could significantly slow down a program that for some reason produces super-voluminous output (e.g. when running a program with heavy debug logging turned on).

Maybe we need better command-line control to override the defaults?  Are there precedents e.g. in Bash flags?