I just been messing around with the new rendering and turned all to max.
I read in the mincraft forum about the incredible render speed so i tried to fly in spectater mode and if the chunck loading could keep up. (It couldn't, but this might be because of the newly generated chuncks).
I went underground and saw that some of the chucks were't correctly rendered. (See screenshot).
Mîght have someting to do with the new rendering.

Hope this helps