All types of aquifer can spawn in the deep dark, causing it to get flooded with water or lava. This can make ancient cities very difficult or impossible to explore once they get added.
To reproduce, use the seed -3945541560037994869 and run the following command:
/execute in minecraft:overworld run tp @p 1004 -23 65
As long as they don't directly intersect ancient cities, aquifers and lava aquifers cause deep dark biomes to be more varied and interesting to explore. Ideally, aquifers shouldn't intersect the structure itself, but structure-less deep dark biomes should still retain this terrain diversity.
(Although, I suppose there potentially could be other issues, like the warden spawning in lava, or glow squids setting off shriekers, that might occur if deep dark biomes contain aquifers.)