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But for content purposes, we can provided a reason below.

However, in an attempt to answer this question from fandom view (as professional producers have yet to post about Voicebank quality).

  • Lily, has been described as fuzzy and nasally. She also has a notable trouble keeping long notes.
  • Utatane Piko, has been described as sounding muffled with odd pronunciations.
  • Kagamine Rin and Len, their first voicebank release was criticized to a point that CFM created an Act2. Though some still insist even Act2 holds similar issues.

Take note that this section was answered based on Vocaloid fandom reaction and shouldn't be taken as the final truth.