The exhaust from the EDF units creates a vortex-like turbulent airflow. Since the exhaust exits over the rear of the wing on the Ho 229, this causes a yawing moment and general instability. The first prototype of this model crashed for this reason.
All successful test flying to date has been done with the extension tubes shown below. It is hoped they can be reduced in size to be less noticable, or eliminated alltogether with the use of stators. This work has not yet been completed. As you can see from the video, the tubes aren't noticeable in flight.