I did some Black Library stuff a few years ago. Not 40k, original fantasy universe, Gotrek & Felix etc. Overall they were a great bunch to work with, and they paid decent money.
My only caveat, with Warhammer or any work for hire: on my last piece I disagreed with the editors about the direction of the story. It’s their world, so except for a few minor concessions, they got what they wanted. When it came out a lot of fans felt the same way I did and said so in their reviews. The thing is, they naturally assumed it was all my doing, when nothing could have been further from the truth.
My long-winded way of saying, the knock on any work for hire can be that at the end of the day, it’s not really yours, but it does have your name on it. Most of the time that’s fine, but when it’s not, it stings a bit.
On balance though, it was a great company to write for and I enjoyed the work a lot.