After playing survival mode in the most recent beta, with custom biome generation off after seeing the update log mentioning it isn't compatible with the new generation, I noticed that ores from addons I had added weren't generating (although I didn't realize another addon had its own generating structures), despite the ores (and structures) intended to be able to generate in vanilla biomes. Later, I started up another world to test whether the creation of custom biome setting affects the ore generation, I saw one addon that I had in my survival suddenly started successfully generating structures, and the ores of the other aforementioned addon also had generated.

How to test:
1. Create two worlds, both with ore/structure generation of their own
2. Make sure the only difference between the two worlds is the state of the creation of custom biomes, one with it enabled, one disabled.
3. Enter each world, and fly/dig around, looking for structures/custom ores.

Observed results:
Ores/structures from addons only generate at all if the "creation of custom biomes" toggle is enabled.

Expected results:
Ores and structures from addons should generate in the respective vanilla biomes, as they aren't their own biomes.