I’m running B4 The Lost City. We’ve only run 1 session, so I don’t have much to tell you about how it plays other than “It was fun” and “The PCs were so busy searching for food and water that they didn’t bother looking for treasure, and only ended up earning about 10xp each for the first session.”
As far as converting it goes, I did end up greatly expanding the actual Lost City. The map of the city provided in the module shows it as being in a cavern just large enough (based on urban population density data from Medieval Demographics Made Easy) to house the city’s current population of 200 families. That doesn’t fit with the backstory of the city having once been much larger and dwindling to its current state. But that has nothing to do with ACKS.
What was interesting is how well the number of leveled NPCs described in the module fits with the expected number of leveled NPCs expected in a 200 family settlement in ACKS, if you assume that the majority of leveled NPCs are members of one of the Lost City’s cult factions.