The borders between each biome are very rounded and smoothed. There's zero noise or "roughness" applied, leading to unrealistic biome shapes and edges, wherein they look like very rounded blobs rather than realistic land shapes.

I've attached two examples showing a snow/ice border being too smooth, and an almost circular mesa blob. (World map screenshots courtesy of the tool MCA Selector). As you can see in these, the biome borders are perfectly smooth and unrealistic. It's most noticable with biomes with contrasting colors, but even the different forests have this problem if you look closely. In the mesa screenshot, you can see some old chunks from 1.17 (it's an upgraded world) and the old mesa's shape is much more rough and realistic.

I've also attached two biome maps from 1.18 and 1.17 to compare (same seed). The old 1.17 biomes (courtesy of Amidst) are very rough and smooth, whereas the 1.18 biomes (courtesy of ChunkBase) are all very smooth and unrealistic blobby shapes.