Whether a speaker is "good" for X, Y or Z genre is always going to be a subjective call. It’s "good for X music" insofar as you enjoy that music on the speaker.
For my kids, apple earbuds are "good" for everything they listen to. There are people who love classical music, but who listen through small speakers utterly incapable of anything like orchestral dynamics...but the musical message is coming through loud and clear for them.
So you always have to take someone’s assessment "X speakers are good for X but not for Y" with a grain of salt, since that only means "I enjoyed X music but not Y music" on the speakers.
This subjectivity plays out with Harbeth speakers like any others. There are those who will say "Harbeth speakers are only good if you like THESE genres of music" while plenty of other Harbeth fans will say they like Harbeths BECAUSE they seem so well balanced they seem to play all genres pleasingly.
I’m in the latter camp. I found Harbeth speakers, e.g. the SuperHL5plus (and even the smaller 30s) to be beautifully balanced, in that all the elements in a mix seemed to come through sounding "right" - nothing shortchanged, nothing over emphasized. That went for all the funk, rock, prog rock etc that I loved, as well as acoustic material. It was all very satisfying.
Now that doesn’t mean I can’t get as much or more pleasure through certain other speakers I like or own. For instance I’m currently using my Thiel 2.7s, and I love the depth of bass, impact and sheer scale they provide. So I do like certain aspects of the Thiel sound over the Harbeth (and visa versa). But that’s not to say I found myself thinking the Harbeths were a failure in any particular genre of music. No more than the fact there are plenty of more powerful speakers than the Thiels means I'm feeling the Thiels are substandard with some genres.
The Thiels rock my world with some Rush I was playing yesterday, with tons of punch and dynamics for my taste. But I’m sure to a horn fanatic they "just don’t do dynamics" as they want, and they’d find some genres unsatisfying on the Thiels, where I enjoy everything. And so it goes...