After seeing a lot of negativity about The Ghost Survivors DLC I'm really loving it. They're brutally tough, as is Fourth Survivor, but I find it really helps give a much better understanding of how the main game should be played at a higher level. It really helps to expose players to strong strategies they can carry into their S+ rank runs and it focuses on the strength of the game by emphasising the need for item conservation, well-placed and considered shots, and using every item in your inventory. I also really like how it showcases the versatility of the weapons. They're very tough (even beating them) but it's very rewarding when you do and I think you really become a lot better at the game by playing the missions.
The only downside I have is although the music in Fourth Survivor carries the feeling of the mission and builds tension, the other missions don't really have an OST that does the same. I still have to beat Katherine's mission (focused on the other two since hers is supposedly the least difficult, and currently have made it to after the basketball court without issue), but I've really enjoyed this so far, especially for free. Mercenaries mode would be good, but this is a lot more in keeping with the game's design in my opinion.