This is coming from someone who loves both. Despite Hyolyn's decline she's still considered a skilled vocalist, whereas Bom didn't have great technique from day 1.
At Hyolyn's peak, she and Luna were arguably the best K-pop vocalists. Over the years she's gotten a bit weaker at controlling higher notes and her airiness is pretty evident, but she is still a better singer than Bom.
EDIT: I was reading the comments and just wanted to say I disagree with people who say she's overrated or not that good :/ Her strain is more noticeable because she has a really light voice and sings in a higher range which is her weak point, but plenty of singers strain more than her. She's good by K-pop standards, and the reason she's praised is mostly for her vocal agility and strength in lower notes.
I think Bom has actually improved... but again she's still below Hyolyn's level. She doesn't deserve any hate for it though, YG told her to sing that way. Plus she's a vocalist that prioritizes style and emotion, I mean she's an amazing artist in that aspect.
People don't attack Hyolyn because she's simply not a bad vocalist. It would be like attacking Mamamoo vocalists who are somewhere around her level currently.