I felt like Luka's users are reducing each years, maybe others non AI voicebank suffers the same issue too? I don't know much about vbs popularity, maybe i have to start watching vocaloid ranking again.
Talking about sound is quite subjective but in the use case, Luka's is like an odd balls among all crypton family with questionable gimmick like EVEC. Yes, she sound unique and no other voicebanks is like her but this apply to other banks too.
Her voicebank is full of issues like inconsistency of phonemes editing, inconsistency of vibrato, lack of smoothness, glitches, audio warping, weird noise in certain performance, excessive fundamental tone, rather poor optimal range handling, poor harmonics handling etc etc. This just scratch only surface though, with all of this issues, the complexity level start to add up very quickly. After you done tuning then you have to deal with her again during mixing and because of her poor harmonics performance, she can be perceived as sound weak or strained and combine with Luka's excessive fundamental tone, her voice can be ear piercing and that's not a good combination.
Either peoples get tired of using her or finding better alternative so it wouldn't surprise me.