Eunyeong (blondie) probably insisted on that either dad or uncle is bad because:
1) he can't stand others being happy when he himself is unhappy. That's hinted in his current arc about his friend. Eunyeong realizes later that he was dismissive of his friend's achievements, because he himself couldn't achieve anything. The realization bothers him and he's trying to change.
2) of his past experiences. Nothing is easy, nothing is free. No family is absolutely happy. So surely Haejoon's uncle and dad are the same?
Although it has to be said, Haejoon's family situation is bothering Eunyeong, because he's concerned for him. So that's that.
hmmm judging eunyung's past experiences, especially with his own father,
1: with hj's uncle, eunyung smelled alcohol on him. that's why he tagged along. his dad is an alcoholic that's why he has a specific impression of people who drink
2: with hj's dad, judging how his own dad was, he probably recognised the signs of abuse. his own mom was probably abused when she tried to protect him or he himself was abused and would hide the injuries behind the clothes.
both of these incidents relate heavily to his own past, that's why he is so quick to judge because (at least now) it's clear that he cares about haejoon. the arc with him not being able to see other people happy is also there but this seems to be different
well to realize that his mother didn't tell anything bad about his father. maybe that's also the reason why the uncle is so opposed to his parents relationship. the fact that he wasn't attentive enough to think why his parents divorced. that sucks. but our Blondie Bhoi realizing that he's sure is good. but the reason why he insists that either the uncle or father may have bad intentions i really don't know what's the reason. is it because he is concerned to a friend? he doesn't want others look stupid? he thinks all parents are shitty? i dunno.haha