I figured. Rin especially sounds good when he tunes her (and I heard Rin is pretty hard to tune but since Mothy's used her for so long thats why he's really good at tuning her)
In my opinion, Len just sounds off no matter how anyone tunes him, though there's a fine line between "he sounds ok it's a bearable off/it's not that noticeable" to full out "unbearable off". Hence why I cannot listen to the original version of Servant of Evil without cringing extremely bad at Len. Then again I'm not one of the OG Vocaloid fans who are perfectly fine with somewhat robotic-sounding voices which defined early Vocaloid.
Miku's just easier to use (if I remember correctly I might be wrong but thats what I have heard) so there's a lot of people who can make her sound good. She still sounds great when tuned by Mothy though *I point to Queen of the Glass, one of my favorite Evillious Miku songs- then again I just like Alice Merry-Go-Round a lot so*