When I didn't own him and only knew him through his posts on YT, I thought he was so awful. Low quality, muffly, not terribly understandable, all that. But after I bought and messed around with him for a bit I found that he actually takes to phoneme edits really well, and can get a nicely fluid sound semi-easily. None of his phonemes are so fucked that I have to spend time experimenting with different combos to make one word sound "right", type deal. And yeah, he's not very clear -- he is muffled, but it's salvageable. Sonika is definitely worse in this aspect. Use some nice EQ on Oliver in your mix and he's fine. It blew my mind when I got my hands on him, but I guess it kind of makes sense. Because Oliver was so popular so quickly, I think a lot of younger and/or inexperience users bought him first-ish, and so it makes sense that he has a lot of "eh" quality uses online. You can't fault users who are still learning for not being perfect right off the bat.
And in his defense, even a lot of long-time Engloid users still don't phoneme edit (or don't do very much), which doesn't flatter the banks any more if the sound quality is already questionable jasdlf;
But yeah. Child's softness paired with not-terribly-crisp mic recording aside, he's a pretty solid bank. On par with other PFX banks. Not as good as Avanna or Cyva, who are probably still the best quality Engloids we got. He's average.
On the other hand...
I adore Sonika but she is probably the worst quality Vocaloid we have, period. Her phonemes are strange and she's muffled and has a lot of background noise. I mean, Sonika has a lot of like? Artifacts. Such as the infamous "Skype sound". You have to edit every word if you want a fluid, understandable (without subtitles) sound from her, and overall just put more work into making her sound good. And then you have to deal with mixing her muffled ass. Again. LOVE Sonika. One of the first handful of banks I bought, and I've done plenty of fanart and all that jazz. But she's just not good