Okay, after listening to a bunch of covers I've formed a more conclusive opinion on her.
Cons:
• After a while Cevio's engine noise really started to grate, she has a buzzy roughness that wasn't present in the Vocaloid version. I'll admit, when I switched from a Cevio song to a Vocaloid song my ears practically breathed a sigh of relief. It's one of those little things that isn't a big deal at first but wears you down over time.
• Her enunciation isn't great, I often can't make what she is singing.
Pros:
• Despite her enunciation issues, she sounds more natural than clearer synths like Cyva. Perhaps in part due to the way her words slur together, she is quite human and emotive. She can pull off a punchy chorus with clarity and emotion.
• The engine noise is reduced significantly in certain songs. I'm not sure if this is due to tuning, or whether the noise is strongest in a certain range.
• I like her tone, how it's not too high or voice acted, which compliments what IA is known for.
I'm definitely going to be checking out more IA English songs in future, focusing on the ones that keep Cevio's engine noise to a minimum. Overall I'm happy with this release, and am looking forward to seeing where she goes next.