For a smoother workflow, you can create sample Root bones for Unreal / Unity from the Tools menu.

Before exporting, please set the appropriate preset in the Tool menu → FBX Export options.

Unity

In Unity, the Up axis is Y. Make sure the Up axis in your rig is also set to Y.

This way, you can verify clean data with zero default values in the engine as well.

Unreal

In Unreal, the Up axis is Z. Make sure the Up axis in your rig is also set to Z.

This way, you can verify clean data with zero default values in the engine as well.