Fine Biome mod. One of the best 10 years ago, but not the best now.

Biome's O' Plenty: One of the most popular mods for a decade+, but hasn't come far since.
II also seems to be pulled in a few directions at once:

Stay the same as it has been for decades.
Turn minecraft into a massive, fantastical, mystical world
Turn minecraft into a realistic, immersive, IRL-esque world.

If they chose just one direction, I think it would be great. The problem is, they are trying to do all three things at once.

Pros:
Some very Realistic Biomes
Grander scale (huge biomes and huge trees)
Strong inspiration for many modern mods.

Cons:
Too many biomes, especially unrealistic
Takes away any "vanilla" minecraft feeling.
Too many new wood types, colors, and foliage. (20 different wood types, all basically the same, completely fill JEI and inventory menus).
Doesn't add other realism or immersion features to biomes (more realistic creatures, crops, or other foliage).

While I love the immersion and realism of Biomes O Plenty, I think that it hasn't grown much in quality, and hasn't changed or improved in years. Still half fake, mystical biomes and the other half are cool, realistic, minecraft-y and IRL-y biomes.

Great for beginner modders and modpacks, but we've all seen it before, and there are better biome mods out there now.