Incompability with Custom Datapacks
Most Datapacks were build for handling vanilla Minecraft worlds. For example you will get flying structures from Custom Structure Datapacks.
Make sure that those Datapacks are compatible with iris (e.g. Made for and/or campatabile with Iris)
Hint: Ask the Developer of those Datapacks if they can be altered to work with Iris
Known non working Datapacks are
  • Terralith