Cyrus is an overhyped villain whose goals don't make any sense. He's little different from Lysandre apart from having a sad childhood...which explains his desire to destroy the world and everyone else in it, including his own team how? I mean, not saying people's bad childhoods don't influence their outlook on life, but we don't actually get to see how Cyrus connects the dots and HOW his childhood informs his goals. It's not like with Guzma whose failure to accomplish the trials and parental abuse caused him to try and lash out at society (all the while giving other underdogs a home), or N whose hatred for Poke Balls and Pokemon battling comes from his childhood exposure to abused Pokemon engineered by Ghetsis. We don't see the actual connection between his goals and his upbringing and neither is it even known to you until you talk to some random NPC.
Everyone agrees that Cyrus's goal doesn't make much sense, but the reason he is super hyped as a villain is because of the fact that he's very cold and emotionless to his own grunts, and the fact that in Platinum and the anime he states that he wishes to create a universe solely for himself, not for Team Galactic. He and Ghetsis are probably the most irredeemable people series, though Cyrus does reform in Adventures. And Platinum did explain that despite being a good student, he did not live up to his parent's expectations and found robots more enjoyable as companions. You are supposed to put it together that he believes that robots are more effective than humans due to lacking emotions that would otherwise interfere with duty.
Hau is underrated as a rival. Seeing this kid who only wants to have fun finally start taking things seriously (but still be happy and friendly) after failing to protect Lillie (IIRC) is great development and seeing him support Hala against the UBs as well as fight you in the final battle of USUM is actually pretty great IMO and he has a dope theme song. I think Pokemon has overplayed the idea that losing is okay and having a rival affirm that winning is still better is a good idea - in the real world, if you don't persevere to improve yourself and try to actually win in things - be it education or health or a career or a relationship - you're gonna struggle a lot. Winning all the time isn't necessary but you gotta win some times. People should know that life isn't gonna be easy or fun 100% of the time and they'll need to be serious at times if they wanna progress or not regress in life.
While USM ruined Lillie, Gladion, Lusamine, Anabel, Looker, Nebby and the Ultra Beasts, you are right they did give Hau a much needed an arc and he's definitely better compared to vanilla SM. I just don't think it was worth butchering other characters.
Silver being abusive to his Pokemon and to other people IMO doesn't get resolved properly enough or even really explained in the proper plot, you needed remakes and FR/LG to tell you that Silver had issues with Giovanni - we aren't even told of their relationship until FR/LG and even then it's more implied than stated outright. You might even miss his redemption arc entirely if you don't rematch him at the E4 (even the Kanto fight happens with a Golbat IIRC). I also think Pokemon threw away a really fun chance at having you team up with your enemy against Team Rocket, your mutual enemy - you both fight them but only separately, never together.
I like Silver and the fact he does change does fit along with HGSS's theme of trusting Pokemon, I agree that a lot of it does happen off screen, and I understand why some people don't like it though. My only real complaint with Silver's backstory is that while HGSS does confirm that he is Giovanni's son, you need an event Celebi to see it, which is currently discontinued.
I suppose while were talking on unpopular opinions on characters, I never understood why people called the Gen 4 evolutions ugly or how they "ruined" the Pokemon. I really like most of them, with Mismagius, Togekiss, Leafeon, Magmortar, Honchkrow, Froslass and Lickylicky being some of my favorites. I don't see how adding cross-evolutions ruined the Pokemon, if anything it made the likes of Yanma, Piloswine, Togetic, and Roselia actually good in battles. The only one I'd really call ugly is Probopass; and while I have heard people call Lickylicky ugly, I disagree. I think its very cute, especially its Platinum sprite, it looks so happy! It reminds me of a puppy who just wants to lick you, though I'm not a dog person in real life, I'm a cat person.
I'm guessing those negative reviews is probably why we didn't get any cross gens with the exception of Sylveon until Gen 8, which is sad, because I've always thought Pokemon like Swoobat, Chatot, Noivern, Lumineon, Skrelp, and Clauncher could use a new evolution. I'm also not a fan of how they handled it in Gen 8. I don't like how they are tied to the regional variant instead of the base form, because the base form of Farfetch'd and Corsola still suck despite their Regional forms getting an evolution.