Nether terrain has many missing features and parity issues, most notably:

1. Lack of nether "ravines", huge tall caverns.
2. Soul Sand Valley nether fossils are MUCH less common. A lot of small soul sand valleys have none.
3. Quadrants for fortress and bastion spawning are much larger than in Java.
4. This leads to fortresses and bastions being VERY far away from spawn. A rough average distance to the nearest fortress would be 600 blocks on bedrock in my experience, and about 300 in Java. This makes obtaining blaze rods and reaching the end much harder. The early game in the nether is much more difficult as a direct result of this. I have stopped playing many survival worlds due to lack of access to a fortress. This harms player retention.
5. Mob spawning was changed in a recent update, but it still is quite bad. Too many ghasts spawn at once even in biomes where they are supposed to be occasional, such as nether wastes.