This signal couples the communication protocol (MAVLink) to the UI layer. On top, QGroundControl uses consistently internally NED for body speeds, and this design pattern forces each and every subscriber to do the coordinate transformation himself, if he can't directly use it (at the point where it becomes pointless to transmit the frame).

A better (and simpler) approach is to change this to velocityChangedNED() and to add signals for other frames as they become available (and emit both).