When forcing a bedrock tree to grow downwards, the trunk will break blocks beneath it.
This can lead to some issues with the new barrier block if players can glitch their way through them.
Also people can cut holes in bedrock to cause players to drop through the world, or dig holes in the bedrock ceiling of the nether.

Steps to replicate:
1 - Place 4 dirt on top of bedrock or barrier.
2 - Grow a dark oak tree.
3 - Break bottom layer of dark oak, and place 4 saplings to grow another.
4 - Repeat step 4 until the trunks grow down and destroy blocks below.

For visual representation of the issue with barrier blocks, I did above steps over barrier blocks as seen in the screen shot.

To see the issue on bedrock, here is a link to a Docm77 video where he demos it: https://www.youtube.com/watch?v=M-gdbqMihAE