PREFACE: This is specifically for the 1.12 version of the mod! Later versions completely rewrote the mod and it might as well be considered an entirely different mod in newer versions.

So, on 1.12 this mod is something you shouldn't touch with a 10 foot pole. While some of the biomes look nice, the compatibility of the mod is terrible. And compared to other biome mods available on this version, it doesn't really add anything substantial besides biomes with very dramatic terrain heights (which are a pain to traverse). The worst offense of the 1.12 version, though, is that it's MCreator. Compatibility with other worldgen mods is terrible, and as with all MCreator mods, it's buggy as all hell. 0/10 wouldn't recommend if not using the rewrite.

So, first of, I would like to emphasize that this review is of the 1.19 versions, however this review may also apply heavily to other versions as well.

To start with the positives:
If you are looking for a mod that adds amazing views, you cannot go wrong with this mod. The mod goes for a more fantastical approach to its content; this means that trees will be large and detailed- The rock formations get a little out there- not everything conforms to realism. For me, personally, this is great because it helps emphasize the escapism Minecraft gives to many.
Many people like to compare biome mods to other popular biome mods. I will not name the other biome mods, nor will I bash them as I believe they all have great content but play to different strengths. I appreciate BYG because not only does it transform your overworld, but also your other dimensions. The Nether gets about 14 biomes (I think?) which is much higher than other mods. For me, this is great because it keeps the Nether interesting, and I never feel like "Why be here when there is so much more in the overworld." because, well, there's just so much in the Nether as well. And more specifically, end biomes. Around 9 or so, but my god does it help with the End. I specifically like the Bulbis Gardens a lot.
The mod has great textures all around. They stick with the artstyle of vanilla quite faithfully, but they aren't afraid of getting out there. It really helps let you feel at home with a "vanilla" tinge but also reminds you that there is stuff out of this world that is out there.
I never get much performance issues with this mod (keep in mind I always use a surplus of performance enhancing mods, however.) But, chunks might be busier due to the increased scale of a lot of things. However, I noticed that biomes with a lot of bush handle better than some mods with similar levels of detail.
My main criticism has to be in the gameplay department. First, the trees. They look spectacular, but, my god you almost can't do anything without a tree cutter mod. If you dislike vanilla large trees, you'll not enjoy cutting these down manually. Besides, I almost don't want to cut them down to build because they're do damn pretty; I feel like I'd ruin it. I dislike biomes with pure grass- take the praries, for example. Now, I know their main gimmick is the tall wheat everywhere (because it's a prarie) But it's just annoying. I can't see anything because the grass is so tall- and I can't build anything because there's so much of it. And I spawn in these often. It kind of stinks, because every biome is beautiful, but a lot of it just feels like fly over. Also, if you find inventory clutter annoying, you may not fair well. The mod adds hundreds- hell maybe even thousands- of new blocks and items. There's nothing wrong with them on their own, but their just so many. Prepare to have your inventory cluttered with different flowers and stones and woods.
Sorry if I went a bit ranty. Nothing is perfect. If this mod were to completely get rid of all my issues, than it wouldn't be quite as awe inspiring to traverse. Any many biomes stick the perfect balance; the Skyris Vale is perfect. The trees are only slightly bigger than Acacia, and there's not much bush. That and I like every block that makes up that biome.
Check this mod out. Even if you don't like survival with this mod, you can hop into creative and fly around for hours.

  (Don't take this into the review; this is me being very nitpicky. The mod developers are working on a cave biome mod called 'caves and critters.' I'm pretty sure. Idk if they still are but they were at one point. My question is: why not just add cave biomes into BYG. It just seems confusing to me why they would make caves their own mod, but not the Nether and End. The caves feel extra boring especially when everything else has been given a makeover and a half.)