Structure related mobs can not spawn in superflat worlds.
This is true for:

• Pillager
• Witches
• Guardians

Steps taken to confirm this:

Forward Confirmation

1. Generated World on any Seed with Superflat (flat) World Generation and included all available structures (default spawn ratio)
2. Using admist and /locate to find nearest structure, teleport to it, and confirm no spawning
3. Save and exit.
4. Using NBTExplorer change world generation type from 'flat' to 'default'
5. Join world and confirm spawning entities
6. Confirm changed world-gen by loading new chunks

Backward Confirmation

1. Generated new default World on any Seed
2. Using /locate to find and teleport to nearest structure. Confirm through admist as well.
3. Save and exit.
4. Using NBTExplorer change world generation type from 'default' to 'flat'
5. Join world and kill all Entities (except player) to confirm no spawning entities.
6. Confirm changed world-gen by loading new chunks

In addition, seed-changes were tested, as well as importing confirmed-working chunks into superflat world and manually creating structures through NBTExplorer.

In all these cases structure entities consistently did not spawn while level.dat contained 'flat' - type world generation.