The problem that black had last time I saw it on hacker news is that it only inserts whitespace. Unlike braces-and-semicolon languages, that is sometimes not enough to format code well. For example, given the following line:

    x[1][4] = a[2] + b[4]

Black will format like either of these:

    x[1][
        4] = a[2] + b[4]
    
    x[1][4] = a[
        2] + b[4]

But not like either of these, unless you insert the brackets yourself:

    x[1][4] = (
        a[2] + b[4])
    
    x[1][4] = (a[2] 
        + b[4])

Maybe this sounds like just one little problem but I think it's a fundamental flaw. I have seen the result of a Python formatter (not Black but had the same problem) applied to a couple of files and it's a total mess. I'd take inconsistent column widths over that any day. I asked the creator of black about it and he was pretty dismissive.