Several Hills biomes do not generate sporadically replacing patches of their regular biome counterparts. They generate like a completely separate large patch biomes, making them quite rare.

Example:

Taiga generates small patches of its hills biome.

But - Swamp doesn't.

Affected Biomes: (May be incomplete)

1. Dark Forest Hills
2. Tall Birch Hills
3. Giant Spruce Taiga Hills
4. Swamp Hills