[Nils Dorigoni]"I heard that with the beam, null to null method, the nulls get lost when moving in/out Z, which is crucial for 3D tracking."
You're both right and wrong in that assertion. the beam effect is indeed a 2d effect (but plexus also is).
You can howerver give the beams 3d "awareness" making the start and end points stay put with the tracked nulls and even make them change size with the distance from the camera (as plexus does automatically). See attached example