I haven't heard a lot of newer commercial Vocaloids, and I tend to like at least one usage I see of an often hated or ignored one. But I remember not liking Galaco, Mayu or Tone Rion (though I really like Yumemi Nemu) for their voice, and Tone Rion's V3 design annoyed me because of the cyber maid little girl look and the stupid useless bandaid on her extremely long boot... sock... thing.
I also historically don't like a lot of Vocaloid Voicebanks for characters I do like. I was a Kaito V3 Straight hater for so many years because I thought V3 Soft was the better V3 bank and V1 was the better Kaito bank for usages that required a bit more power. Turns out that V3 Straight can be really good if utilized properly, and I eventually found really good usages using him. While I have grown out of that, a current day upset for me is Luka + Meiko SP. as someone who loves the two's unique vocal properties, it makes me so sad that so many very experienced users have gone on to say they can't get them to not sound like Miku :( such a dissapointing change of direction to me.
When I was young and new to the fanbase, I would pick a Vocaloid I thought was "overrated" and tried to be a hater because I thought it was very cool to have negative opinions on fictional characters. The problem with that is I chose other characters from other series to hate based on if i found them annoying in terms of personality, something that can't be really done with vocal synths outside of aesthetic reasons (like not liking their voice or design). As such, my least favorites outside of the mentioned "I don't really like their voice or design" crew were founded on nitpicks.
I remember Len being one such Vocaloid because i was a ride or die Kaito fan and thought Kaito deserved more popularity. A year later, I would aim my obligatory hate lazer at Utatane Piko, because a lot of the mods on Vocaloid Amino loved him and some of the userbase would cater to that love to get their creative posts featured on the front page. Since Piko and Hiyama Kiyoteru's (+ Yuki's and Miki's) birthdays are so close together, I ended up thinking Piko was being unfavoritely favored over Kiyoteru (a character and voicebank I adore dearly). Interesting to note that I loved (and still really like) both these guy's voicebanks. I don't know what 15 year old me's problem was.