This is definitely the case for Cangel and Spuffy. And Angel and Buffy were screwed over for these two ships.
I'd say Spike and Cordelia got screwed over as well.
Spike was a fantastic character in Season 2 and Lover's Walk. The Harsh Light of Day and In the Dark are the last time we see pre-neutered Spike. However little by little the writers started chipping away at the character. He was more or less comic relief in Season 4 and that worked fine (though the chip storyline should've been treated more seriously than just an impotence joke), but by the time we get to Season 5, Spike's storyline is literally just about Spuffy and this continues throughout the rest of the series. Spike is my favourite character (because of how great he was in Season 2) but I would've been happy to see him less (so that it would always be nice when he did so up) so long as they could've kept what made him great in Season 2. He should've stayed with Drusilla and they should've both been in Angel for the Darla plot. Not only would we get to see all of the Whirlwind together in present day but it would cause Angel a dilemma as he is split between his old family (The Whirlwind) and his new family (Cordelia, Wesley and Gunn). Spike and Dru should've been used to advance Angel's story rather than Dru just being tossed aside and Spike being forced to be Buffy's lapdog.
Cordelia's character was just completely destroyed, first by Cangel then by the Connor plot. Cordelia used to have a confidence to her where she was nobody's fool. She was humbled by her experience in LA which made her less uppity towards people of lower income since she was now one of those people. This was good character development. She learnt to work as a team and care about other people but then Cangel happened. The real Cordelia would never settle for being second best to someone else. She knows Angel's heart belongs to Buffy and always will. She was there when Angel lost his soul and went evil and killed Jenny, she was worried that he could enter her car because she invited him in one time and wanted to de-invite him from her car, the point is she knows what would happen if Angel had one moment of perfect happiness. For Cordy to be in a relationship with Angel, she's either willingly risking bringing forth Angelus despite witnessing the consequences of that first hand or she's settling for being second best. Neither of which the real Cordelia would do. Cordy then went blonde which looked awful and also has the added connotation that Cordy is changing herself to be more like Buffy in appearance, which again the real Cordelia would never do. The writers tried to big up Cangel as being on the same level as Bangel but the comparison just falls flat because Cordelia is not Buffy and is nothing like her. Trying to force Cordelia to be more like Buffy is just ruining Cordy's character. Then came the Connor plot and what little dignity Cordy had left then just got trampled to death.
Buffy freefalling after her resurrection in Season 6 wasn't a bad storyline in itself, it shows consequences to such a big spell. Her confiding in Spike was a nice touch since he would be the only one who knows what it's like to come back from the dead. However, the sexual "relationship" was just awful and not something Buffy would do even if she was depressed. In "Anne" when she was falling apart after killing Angel and sending him to hell, she couldn't stand to be around her friends but she never jumped into bed with someone she didn't love to "feel" something. Xander and Willow were screwed over in Season 6 with the way they just left Buffy to it after her resurrection. They were worried about her all over the summer between Seasons 2&3 yet when she starts displaying withdrawn behaviour again, they couldn't care less. Xander and Willow were never that self absorbed.
Giles just leaving because he thought Buffy needed to "grow up" was another wildly OOC moment. In Season 5, he was going to leave because he thought he wasn't needed but then stuck around when he realised he was. In Season 6, he was needed more than ever, even more so than in Season 5, yet he decided to leave anyway. Obviously ASH wanted time off so Giles had to be out of the picture, but the writers could've given a better reason for why he had to leave, like maybe there were reports of the potentials being killed off and Giles had to go back to England to assist. He'd tell Buffy he was just a phone call away if she needed him, however when Buffy calls it's a bad connection and she can't get through. Right there you've got Giles out of the picture without him completely abandoning Buffy in her hour of need.
With Angel, I don't think he'd want to enter a relationship given the fact that if he has one moment of perfect happiness then he'll lose his soul and become Angelus. Bangel caught him by surprise so he was too involved there before he realised the risk of losing his soul, however post Buffy Angel knows what might happen. If he doesn't want to lose his soul, then he'd be constantly holding back emotionally which isn't fair on the other person. Angel wouldn't want either situation to happen. Besides the fact that he still loves Buffy, he can't allow himself to fully love someone else or else he risks becoming Angelus.
I love Angel/Darla for it's complexity. I'm fine with Buffy/Riley and Angel/Nina.
Dangel is the only Angel ship that could rival Bangel in my opinion, it's such a natural triangle as Darla represents the darkness of Angel's past whilst Buffy represents the light of his future. Being with Buffy after Shanshuing is ANgel's light at the end of the tunnel. It's natural that the vampire in ANgel would want to return to Darla whilst Angel's soul wants to be with Buffy. There is an organic conflict there where both ships make sense and Angel could go either way depending on the day he had.
Angel/Nina (Nangel?) doesn't have the punch to it that Bangel and Dangel has. Firstly, Nina isn't a "big" character so in the grand scheme of things she's insignificant. Secondly, she was only introduced in the final season of Angel, sometimes it takes the audience a while to take a character to heart so there was no way she was going to compare to Buffy or Darla who the audience had a lot more time to love. Kennedy suffers the same problem in Buffy Season 7. She was introduced in the last season and was given importance despite not earning it at all. The scene where Buffy gets kicked out of her own house could've been absolutely heart breaking (though still annoying since it was her house) if it had just involved the Scoobies. However, because Kennedy initiated it (and all because she wanted a bigger say as she later admitted to Faith) it was infuriating because Kennedy had neither the right nor the status to do that.
Briley was okay in Season 4. However, his behaviour towards her is concerning in Season 5 and makes his actions in the Yoko Factor look even worse than they did originally. In Season 5, Riley is all about control and wanting Buffy to "need" him.