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IOSYS and Kagame are both hit or miss, they have very specific creative specialties that don't work with everyone (I don't see them meshing well with Suisei, for example). Plenty of holos have tried over the years to get big hits out of IOSYS but never surpassed, or really even came close to, Ahoy. Ui(9)+IOSYS+Kagame simply struck a perfect formula together that far surpassed what any of them were capable of alone.
In particular both IOSYS and Kagame tend to default to a pretty safe otaku-moe style, even in the older stuff they're more well known for, and kind of rely on their skill and reputation to stand out. Although they are very good at what they do, when paired with more conventional vtubers it produces an also conventional performance as most vtubers just want some cutesy off-jpop with memes mixed in. IOSYS did Akutan Suki Sugi and it's the most boring vanilla shit I've ever heard. Ui is the one who changed this pattern radically by giving them something really fucking edgy and sexual to make instead of the usual cutesy personalized pop song.
Basically, Ui(9) gave them a unique attitude that isn't present in most of their other works. This had a cascading effect where D.watt produced a different tone of instrumental to suit that attitude (which was very good) and Kagame atypically drew a sexually aggressive dancing loli (which was very good) and Maron wrote a bunch of lyrics that half of hololive are terrified to recite to this day and ENs are literally banned from singing. All of this happened because of Ui being a flippant lolicon with a depraved fandom who were the main target of the song.