I have played the 1.12 version of this mod the most, that said this review will also take into account newly-added improvements and content added over the years

What is it ?

Biomes o' plenty is a mod adding new biomes, blocks and decorations to your world;
This means new trees, flowers and mushrooms, all found in vanilla biomes, newly-added biomes and even the nether and end.

What does it offer ?

New biomes to explore integrated with vanilla minecraft's biomes. The new biomes add trees which resemble the way vanilla trees generate, and the decorations including custom tallgrass, mushrooms, thorns, flowers and more are also added to the vanilla biomes for consistency, which makes all of them look a bit more detailed when traveling on foot. Most of the added biomes emulate real-life environments adapted to minecraft's style, but there's quite a decent amount of interesting outliers such as the new rare safe haven islands such as the Flower Island or the Origin Island , and the magical biomes which include the Mystic Grove and the Ominous Woods .
This mod also houses unique nether content which also includes whole new biomes/blocks/decorations, even all the way back in 1.12 which lacked nether biomes other than the default "hell" one. All these biomes are often compatible with other biome mods in versions past 1.12, but there still were some amount of intended compatibility back then such as with NetherEx for the nether generation.
The newer versions bring more end content as they have reinterpreted this dimension as some sort of dead ocean featuring ghostly-jellyfish structures, algae-covered forests and large empty buildings reinforcing the weird wasteland vibe. (in 1.12, there is some amount of end content, but it is mostly ores and a custom end glowstone block; the spectral moss doesn't actually naturally generate same for the end plant(s)/tree)

Flaws ? Loss of content :

Since a long time it seems biomes o' plenty has lost content update after update and it seems a lot more focused with being available on as many versions as possible than keeping everything in place. While it is true new biomes and blocks were added since it's 1.12 version, we also lost a bit of both .
Another thing which was present in 1.12 but is lacking in recent versions such as 1.21.11 is the highly-customizable config; While the config files are still here, we can no longer tweak major aspects of the world generation through them nor through the interface before starting a world since this one no longer exists (that means no more easy biome-height tweaking, no continental worldgen option, among other things).

In conclusion

This mod is genuinely a good one on a lot of fronts as it adds new biomes, blocks to those biomes as well as decorations which can be found both in vanilla and modded biomes; That said, the world generation with this mod is pretty basic in comparison to its current competitors in recent versions, and when comparing the mod in its current state to its 1.12 or even earlier iterations it seems there isn't a lot of new and interesting content added, I would even go as far as to say it lost a bunch of exclusive content (biome-specific gems, biome finder, worldgen quirks such as the nether hives, and even more when you look at older versions of biomes o' plenty). It is still important to admit that some of these losses are due to minecraft directly implementing them in the base game (such as mud, bamboo and fallen leaves).
If you want a mod which adds new content and make sure it fits with the vanilla word generation, then this mod might be for you, but if you're looking for more drastic changes or more interesting new content added to biomes, then you may find this mod's additions a bit boring.