IMHO, GGN was sitting on a goldmine with the twins. If I recall correctly, they were getting into 3-view territory when the project got axed. Their manager were planning all sorts of stuffs to capitalize on their popularity but alas, it wasn't meant to be.
For the others, Bae was miscast on MyHolo (her voice was cringe and not natural compared to Council/Promise) same with Okayu and Korone where they just played with games all the time and their models was shit. Still, this was very early in their vtubing careers so ofcourse it isn't as comparable to now.
The biggest corpo fuckup for me is Polka when she was Nobuhime Oda. She was getting popular with her covers gaining a million views (though I don't know if this was before or after she went to Holo) and has an INFAMOUS mochi eating ASMR video. Not surprised she graduated because her previous corpo was Upd8, known for major fuckups and mismanagement.