1. I like OP characters because it's nice seeing them just smash through everything. At this point, I am bored of it though.
2. I don't like semes who rape and bully the uke. I like sadistic ukes. lel.
Honestly i believe that by them making mistakes not only does it portray them as more "human" in the sense that we can somehow relate to the character, but also, imo, gives a bigger feeling, like, you get mad at the characters bad choices and you feel happy when they overcome their hardships together.
About your second question, i find that more and more yaoi mangas (can't talk about the other genres as i don't usually read them) just have rape and the uke just, gives in, and falls for the seme, and i guess i'm just tired of that. Especially in omegaverses, that happens A LOT, it's rare for me to read an actually good omegaverse.
I am up for a sadistic seme or a sadistic uke, but i just feel like if they try to force themselves on the other, they shouldn't just be like "oh, he raped me, now i'm madly in love with him", i want them to have consequences to their actions, doesn't matter how small they are, there should be some consequence, or at LEAST give a good reason for the partner to fall for the person that raped them.
I also prefer it when a character has some flaws (as long as it isn't acting, as one might say, like a dirty old man around people of the opposite sex). Stories where the MC is too OP or perfect usually bore me or occasionally have made me jump around the house in aggravation.
As to why people enjoy them, I think people just like the moment where someone completely annihilates an enemy/bully/relative who was ruling over them and/or others for so long. Something that we rarely get to do in our own lives.
As to why people often bash on more realistic characters, I personally believe it's because they plainly seeing the MC's flaws without any shroud of their own bias or a connection between the story and their own experiences. For example, when I drive and someone is speeding along and cutting people off, I often like to assume that they may have a reason because I know there have been/will be times of emergency in my life and I may or may not drive like that when those times come. However, when I'm in a friend's car and the same thing happens, any comment about it from them is usually negative because they assume that the person is simply being rude, rather than considering any possible circumstances.
It's the same with fictional stories. We often don't consider the circumstances that a character is truly under when making their choices (emotional duress, obviously not knowing as much as the plot as us, trauma, lack of confidence, overconfidence, desperation, etc.).
I could write more on this topic as it is one I do feel strongly on, but it then might end up turning from a rant into a mini essay :)
Why do you want the perfect character in webtoon and manga, I mean I get wanting the perfect life (in manga and or webtoon) because that's obvious but why the perfect character with no flaws? Besides I'm too much of a good guy 'flaw' I mean it's pretty hard to find a character that screws up much and all, (it's hard to find a well written shounen ai story and all) and the one's that are well written get crapped on because the character screwed up it's not like there human (I know I'm talking about fictional characters and all trying to make a point.) My Question is why? Do you want the character to never make a mistake? Do you not want them to have regrets? and Why is it certain characters are okay? Semes mostly, even in shounen ai the seme could rape or threaten the uke somehow it could seem justified but if the Uke even lifted a finger they'd be the bad guy is there a reason for that?