Datafixers will not run on anything holding itemstacks (drives, any machines holding stacks), meaning any stack format changes made by Mojang or other modders will not be run.

Details:
https://www.reddit.com/r/feedthebeast/comments/7fbqfw/psa_modders_are_you_calling_datafixers_on_your/

Hi! Thanks for the suggestion. This has been added to the RS 2 tracker, and will be implemented for RS 2.x.

Closing this issue for RS 1.x on this repository.

Please read my comment at https://github.com/MinecraftForge/Documentation/pull/142/files#r155961985