I'd like to add my last experience with GHS Black Nylon, because I couldn't find many posts about them.
first of all I have to mention that I do not like fluorocarbons, they are too brittle sounding, and I like my ukulele to sound mellow but clear, and by far fluorocarbons didn't met my taste.
as someone mentioned years ago, not every string works on every instrument.
i probably met the GHS for the first time many years ago on my first ukulele, a 30€ laminated Arrow with black strings. I liked how it sounded, but it was a cheap ukulele that, with Aquila's (tried after like 7 years without changing strings) sounded "better", or I should say "louder".
I still have that uke with Aquilas, but I gifted it to a friend of mine
however, I then bought an Ohana SK38, when the fretboard was still rosewood, and tried many strings on it, but sold it after a while and bought a Mahimahi S3 on which I also tried many strings and ended up keeping it with New Nylgut in 2017, and still strung with them.
in December 2022 I bought a Kiwaya KTS4, same "martin style" ukulele, that came stock with GHS H10 black nylons, and I liked how it sounded, but thought that it had less volume and projection compared to the Mahimahi, but then I recorded it:
I still like the Aquila on the Mahimahi, even if they are a bit muddy, but has a percussive response that I like, so I tried Aquila on the Kiwaya and... didn't like them in any way!
so I came back to GHS, and I love how that uke sounds with them.
I also expanded my instrumentation with a Kiwaya KTC1 (found used in Milan, as new), still with the GHS on, and it sounds so sweet that I never thought of changing them.
the next one is the following Mahimahi T3, the tenor version of my soprano: it came with Aquila NN, and it sounded awful... those strings on a tenor are high tension, tight, and with no sustain at all.
I had a set of GHS H10 (soprano/concert gauge), and Kiwaya suggested me to try them, as they use on their tenors (the GHS tenor set has the roundwound C), and I LOVED them on this uke.
here's how it sounds, hope you can see it on instagram (if not, tell me and I'll post in on youtube).
59 likes, 3 comments - pierfra_san_ su January 6, 2023: "“Gli Uccelli” del maestro #francobattiato adattamento per #ukulele . . . . . #music #instamusic #cantautorato #cantautoriitaliani #i..."
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I love the clear but mellow sound. those strings let every string "ring" as they should, but without excessive brightness and harshness. they also have a perfect intonation on every string, and work well both strumming and fingerpicking.
another thing I love is that they are soft, well balanced in tension, but still with plenty of "drive", so the volume is not less on these solid mahogany ukes.
so +1 for them, but said as a player that doesn't love the fluorocarbons