Unless I'm doing something wrong, it seems that loaders and unloaders won't interact with storage crate minecarts from Immersive Engineering, and loaders delete liquids that they try to put into barrel minecarts without actually putting the liquid in the minecart.

Minecraft 1.16.5
Forge 36.2.4
Mods:
Immersive Engineering 5.0.3-138
More Minecarts 1.3.10

The code currently just uses the Forge API for entities with item slots and fluid containers. I'm not really sure if this is my issue or Immersive Engineering's. I will not be adding individual mod compatibility at this time.