Darkest Dungeon is a dungeon crawler with unforgiving difficulty and heavy on RNG, but you probably already knew that. Despite the intimidating looks, it's not a complicated game mechanically, and you can't even lose unless you play with the optional time or deaths limit, just remember than recovering health is easier than relieving stress, don't take stupid risks (also known as being overconfident) and bring the right tools and heroes for every mission. The game has been patched since release to have a smoother difficulty curve as well. As for the RNG, that part is true, I hope you like gambling because that is the level of randomness you should expect.
Also, almost every action is accompanied by a line from your ancestor who plays the role of narrator, not only he does an excellent job, but his lines are insanely quotable. That was a genius move from the developers, it makes this game very memorable.
I haven't played the DLC expansions myself, but the base game alone is like 50 hours long on average. If anything, I think the game is too grindy, but apart from that I enjoyed my time with it.