I made a copy of a few worlds, all initially created in different versions and updated them to check out the biome blending. 

Attached are some images of bugs, but the list I've found so far include:

• Trees being generated out of dirt, stone, or terracotta.
• Terracotta being generated underwater, either in rivers or oceans (this seems to be due to the biome matching elevation with a previous ocean or river, but not generating the correct material under the water)

• Rain underground in existing chunks, but not in newly generated ones
• Some borders not matching the terrain 
• Some structures generating either a layer too low, removing grass, or missing a layer of grass (one of the image seems to be swamp generation being used, while the other I'm unsure about)

Overall, this stuff is looking really good! Hats off to everyone at Mojang that made this possible! 

Edit:
Grum asked if I could provide a world downlead, but I unfortunately can't add it directly to here. Instead, here are some drive downloads for the world.

Plasma World
https://drive.google.com/file/d/1y_9W0vY0yJWMLzP1mnYyHcVFm6qvHGzu/view?usp=sharing

Issue Locations:
-204 80 2802
201 80 2855

Phoenix World
https://drive.google.com/file/d/1D0Me2x5Xcspuu7IlrBm81NPCESp_WBSJ/view?usp=sharing
-1170 90 -700
-1822 90 -446
-2291 70 200
-4225 90 -682

It seems grass and flowers are also being replaced with terrain blocks. Hope this helps Grum!