Not in detail, but he was widely rumoured to be dodgy. Not explicitly in terms of sexual assault - the way it was usually put was in terms of him having groupies, or weird behaviour around female fans.
That in itself was unusual, because most authors aren't well known publicly, many of the top writers in genre fiction are women and obviously don't carry on like that, and even among male writers there aren't many with his profile.
I also think, and this would make me very unpopular on the subreddit, that he's quite different to Warren Ellis. Ellis is an awful sleaze, but most of his bad behaviour - which is 20 years in the past - was transactional sex with aspiring writers or artists who thought he'd make introductions and get them a career in comics. I believe the women in his case because nobody comes out of their stories looking good.
But Ellis was just a man doing what men do, using his status in the industry to go to bed with women who thought they could use him to advance their careers. He wasn't the writer with a reputation for constantly trying to cop off with barely legal goth girls. That was Neil.