To answer the first part, people on 1DFF hated Brittany from Forever Like This, and the prequel story (which was purely 1D), I Wish. It was a basic love triangle premise - Brittany was dating Harry and Niall had this creepy obsession with her... and people hated her! It was so funny to me because Brittany, though not perfect, didn't do anything nearly as bad as Niall did in that story, but no one said a bad word about him LOL. That kind of answers the other question as well, but he was king douchebag in that story, going behind Harry's back, kissing and confessing his love to his best friend's girlfriend, and then when she told him no, not leaving her the hell alone.
But people hated Brittany lol
Some readers were not cool with how I portrayed Niall in other ways in that story. They didn't have a problem with him being completely selfish in the Brittany situation, but they did have a problem with me establishing his character as kind of womanizing.
I stopped reading most 1D fic except for a few authors who I know will write quality stuff because of this issue - I can't stand it when the guys are out of character and super one dimensional. There are a few BSB stories that I have read where the guys don't act like themselves - I find this with Nick in particular, but that could also just be because I mostly read Nick-centered fic. But for Nick to stop being womanizing and settle down, I find that I need there to be a reason for that. I can't picture him as anything but the nerdy, vulgar popstar I see him as in real life. For me to enjoy a story and see Nick differently, I need a transformation from that to happen within the story, not just be there right at the beginning. I need to watch him change from his recognizable self and grow from that as a result of what happens to him in the story.
I wasn't thinking of any story in particular in that wall of text lol, just the endless Nick stories I have clicked on in my life, because I shamelessly love Nick romance lol.