Blocks from Oh, The Biomes You'll Go do not generate in the nether using Terralith's terrain generation. Biomes still appear, but the only hint that they even exist are some grasses scattered and maybe some biome-centric block columns. None of the trees or custom nylium generate.
After some isolation testing I've found it's strictly a conflict between BYG and Terralith. I'm sure there's some way to fix it but I'd like some help in that regard