What is it ?

A full revamp of four biomes (+ a few smaller features on beach biomes) including aesthetic improvements through diverse and often useful flora, fauna with interesting mechanics and related items, building blocks and features adding content to other systems such as enchantment or redstone;
There is also one added structure for each biome overhaul.

What does it offer ?

A complete vanilla-style update on four biomes : Mushroom islands , Badlands , Swamps and Roofed forests

Spoilers down here for the first biome they improved :
The Mushroom islands ,
In this biome upgrade they added glowshrooms underground, lots of decorative mushroom patches, plenty of mushroom related building blocks (+ new white tree as well, added way before the pale forest was introduced to vanilla minecraft) as well as two new entities being the glowfish and the mushroom trader. Beside that there simply isn't much to add to the mushroom island revamp and since it is a biome on the rarer side it makes sense to me why they might not want to lock away more interesting features for those who have yet to find it .

Spoilers down here for the second biome they improved :
The Badlands ,
In this biome upgrade was added cowboy pillagers, ghost towns with a new poltergeist block acting as a redstone randomizer through possession of nearby blocks which I find a cool take on this concept, terracotta decorative blocks, new cactis and a little critter which allows for the craft to an antidote against poison. A nice detail I like and isn't mentioned often is the dirt variant near badlands lakes, containing gold nuggets at times; it really gives you a taste of the amount of gold you will find in caves under this biome

Spoilers down here for the third biome they improved :
The Swamps ,
In this biome upgrade (which btw they did before Mojang's wild update like many) they added sunken witch ruins, two new types of exclusive swamp trees being the classic willow tree and the cool swamp cypress (anew flower is here too); they added peat, which is a both a good block to dry and use then as a furnace fuel as well as the best ground to use to grow crops on. They also added a bunch of critters in this biome, such as the useful lightning bug, to be used as a light source or a throwable electric flask depending on the method for its collection; That said beware as the swamps house a surprise within its sometimes deep pools : the decayed, a shield wielding undead using the depths to its advantage. The best feature for this update has to be the witch quest system : by disguising yourself as a witch you can accept item fetch quests for rewards, with the rarer the items the better the gift, including exclusive potions combining sometimes several stronger versions of vanilla potion effects in one bottle

Spoilers down here for the fourth biome they improved :
The Roofed forest ,
In this biome was added some new flowers, a glowy one being added on dark oak leaves with itchy ivy slowing you down, discouraging people from traveling above the trees and producing enough light to attract a new hostile flying creature being the moth. Small cute plant-like living creatures are added in the forest and serve as a sort of crop/mob which can get you most if not all types of dyes in minecraft. There's owls too. Crystal clusters can be found on the biome's surface, those crystals are needed for an a new take on going beyond the enchantement max levels which does not involve an anvil (unlike quark's ancient tome system which isn't bad, but honestly the anvil really needs a rework), as a random enchantement on an item can be improved past its limits at the cost of of a random curse (with new ones added by the mod too, and probably compatible with modded curses). Changes were made to pillagers too, as they rarely can now drop crude fragments and crude cladding which can be used in one of two ways : upgrading minecraft's leather armor into a cladded armor, reducing and sometimes ignoring arrows and projectile related damage. the second use of crude cladding is for making stone golems out of cladded stone, those golems being crossbow wielding turrets. One thing I have to say is that unfortunately their aim is not that good and they take their time between each shot, making them not that useful for the cost they require. Instead of a new structure, this biome actually replaced the woodland mansion with its own overhauled version, which contains some of the things mentioned earlier as well as new content that is only found there : a special decoration type similar to banners is found here, depicting mostly minecraft mobs. Now not only the mansion houses a few floors, but it also contains a dark basement with a new boss at the end; this boss in terms of attacks and behaviors really seem like a "vanilla +" boss and doesn't go too crazy with his patterns, but it is nice to finally see a new basic illager boss which isn't tied to raids in any way

This mod's performance is good, its content are to me really interesting and add a lot to their respective biomes.

In conclusion

So far my favorite content has to be what's been added for the fourth biome as it is clearly the one with the most in terms of features, and I do hope somebody else makes a fork of this mod and make the desert update they were planning on adding before stopping updates (check their discord server for info). I really love this mod, and it is probably my favorite fabric mod (even though this mod is now available on forge too)
They have their own official wiki detailing everything one might want to know if advancements aren't enough so that's always a plus in my books.
I would like to finally precise that the fact this mod covers these four biomes stated earlier makes it quite compatible with Ecologics, as they add content to different biomes and both do it in a vanilla+ style (throw-in Environmental for the best biome-improvement experience, though they will have overlapping content which may break your immersion a little)