Currently when you bonemeal nylium, you get a distribution of nether vegetation within a 5x5 block area centred on the bonemealed block.

This vegetation for warped includes warped fungus, warped roots, crimson fungus and crimson roots as well as nether sprouts and twisting vines (these generate with a different method to fungus and roots though).

As for crimson it only generates crimson fungus, crimson roots and warped fungus, but not warped roots.

The below table highlights the weightings (found through yarn mappings) of each type of foliage for both warped and crimson nylium.

As you can see, the crimson nylium weightings add up to 0.99, not 1, and I believe this missing 1% belongs to warped roots as that would add an element of symmetry to both types of nylium while ensuring the weightings of crimson nylium are mathematically precise. 

Related code:

public static final WeightedBlockStateProvider CRIMSON_FOREST_VEGETATION_PROVIDER = new WeightedBlockStateProvider(DataPool.builder().add(Blocks.CRIMSON_ROOTS.getDefaultState(), 87).add(Blocks.CRIMSON_FUNGUS.getDefaultState(), 11).add(Blocks.WARPED_FUNGUS.getDefaultState(), 1));

public static final WeightedBlockStateProvider WARPED_FOREST_VEGETATION_PROVIDER = new WeightedBlockStateProvider(DataPool.builder().add(Blocks.WARPED_ROOTS.getDefaultState(), 85).add(Blocks.CRIMSON_ROOTS.getDefaultState(), 1).add(Blocks.WARPED_FUNGUS.getDefaultState(), 13).add(Blocks.CRIMSON_FUNGUS.getDefaultState(), 1));