Judging by how the Mega Dungeon is designed, and the default config options, it looks to me like it's intended to generate in the overworld, as long as it's not in a beach or ocean. As it stands, it generates in all modded dimensions, which may be fine in some cases, but dimensions with floating islands or otherwise lacking ground wouldn't fit well, if at all.

A proposed solution would be to have a config that says what dimensions the structure is added to. Since every dimension has its own storage for what biomes a structure can generate in, adding a dimension check during BigDungeonModule.addDimensionalSpacing may be a possible way of handling it.

This also occurs with Vanilla dimensions in 1.16.5.

This also occurs with Dimensional Dungeons 1.14.0 in 1.18.1. The dimension here is tagged as VOID instead of END as it is not supposed to generate in the end

I'm the creator of Dimensional Dungeons. I gave jarquafelmu some wrong information. My custom biome (dimdungeons:biome_dungeon) has a category of "none" not "VOID". But "none" matches the category of the vanilla biome "minecraft:the_void".

the_void
biome_dungeon

I think you should consider adding "none" to your default category blacklist, along with beach, ocean, nether, and end.

For what it's worth I peaked at a 1.19 snapshot and it looks like Mojang removed biome categories from the json. :( So we'll both have to find some other way to convey this information in our upcoming ports.