This summary of a YouTube video about him explains it pretty well:

Rick Berman took over showrunning partway through S1 of TNG when Gene Roddenberry's health declined. He only had one real producing credit to his name, and it was for a children's show. Some of the crappy things he did included:

• Being awful to women actors, corroborated by Denise Crosby, Marina Sirtis, Terry Farrell, Jeri Ryan. He gave them zero respect during contract negotiations, increasingly wanted to sexualize their roles (catsuits, "decontamination" scenes, etc), and straight up fired Terry Farrell from DS9 after she just asked for an episode reduction for S7.

• Screwing over Wil Wheaton by forcing him to turn down an opportunity to star in a big movie for no reason other than control/power

• Denying Garrett Wang the opportunity to direct an episode of Voyager, even after a bunch of main Trek actors have directed episodes before

• Attached his own name to the DS9 episode that introduced the Maquis, despite not having anything to do with the episode, because he was afraid Ira Behr would want royalties for VOY (since the Maquis were integral to VOY). Ira had no intention of this, it was Berman being paranoid and having a lack of trust.

• Consciously and continuously steering away from any references to gay people, despite numerous good script pitches and actor pitches. Notably, had completely genuine, organic opportunities to turn Garak and Malcolm Reed gay with full support of the actors, but instead completely steered the other way and put them in situations to "confirm" their straightness.

• A general attitude toward safeness and dialing back any bold actions. DS9 writers basically had to go behind his back to do certain things, and he hated the show as a result

• Also hated the idea of serialized storytelling because he believed all the money was in syndication, where single episodes needed to stand on their own.

• Told the human actors on VOY to intentionally be boring so that the alien actors would seem more realistic. Numerous scenes had to be reshot because the humans seemed too emotional.

• Increasingly stuck his nose in the writing credits for VOY and ENT (even though he wasn't a writer!) as the franchise began to spiral downward. Once he gave showrunning control over to Manny Coto for S4 of ENT, the show became good, although by then it was too late.

https://gamefaqs.gamespot.com/boards/233-star-trek/78369089