I have played this mod in 1.19 with friends on a modded fabric server a while ago, I didn't know anything about the mod before that and it was selected as the main worldgen mod a bit randomly after I watched a horror video using it; My experience and point of view towards this mod is mostly tied to this experience, even if I just tried for a bit the 1.21 version before making this review

What is it ?

Regions Unexplored is a mod which adds a lot of new biomes (about 70 as of the 19th of July 2026) to the usual overworld and nether worldgen, with a lot of blocks to go with them.

What does it offer ?

New biomes with extremely detailled flora, rocks and aesthetic thingies which brings them to life. The environments are both realistic and of the fantasy type, with a lot of variety in the blocks and decorations used depending on the biome.
When I say "new biomes" I mean that with this mod there will be breathtaking overworld biomes, lushier cave biomes (list : Ancient Delta, Bioshroom Caves, Prismachasm, Redstone Caves, Scorching Caves ) and weirdly-alien hellish biomes for the Nether too (list : Blackstone Basin, Glistering Meadow, Infernal Holt, Mycotoxic Undergrowth, Redstone Abyss ).
There's also one new aquatic biomes as well.

In terms of flaws I had noticed all the way back in 1.19 that having this mod generate biomes in your world seemed to reduce greatly the frequency of vanilla biomes (which can be a problem if you have content, modded or vanilla, that is only tied to specific vanilla biomes); It seems to no longer be so much of an issue in more recent versions of the mod, and the configs are also pretty easy to figure out, which is great since you can decide inside them which biomes appear and chances of replacement of a biome by another selected one.
A second issue that I found besides some missing crafting recipes which have been added since is the fact that the new biomes often seem so detailed that the vanilla biomes not receiving any changes in look whatsoever makes them feel out of place :
Thankfully I have just now heard of "Regions Unexplored: Expansion" which seems to fix that issue (Look at the reviews for this separate mods for more info on the goods and bads and a datapack-alternative that another reviewer wrote about), and it seems there is still some amount of decorations which gets automatically added to some vanilla biomes.

In conclusion

This mod is as of now my favorite in terms of worldgen mods which add new biomes, with the biome concepts and decorations being the main reasons why (check the mod's page for prettier pictures). One of its strengths may come from the amount of addons which exist to create compatibility bridges between this mod and other well-known ones.
It is pretty similar to biomes o' plenty in the sense that if you want a mod which adds new content and makes sure it fits with the vanilla word generation, then this mod might be for you, but if you're looking for more drastic changes , then you might find this mod's additions less interesting and would probably favour mods like Terralith or Oh The Biomes We've Gone instead.
Globally I find this mod much better than biomes o' plenty but I still wish the various decorations had more utility, I always find it interesting when even the most ambient of blocks and entities have alternate uses outside of the vibe they participate in creating. Still, this is an amazing mod , goes well in a larger pack.