@tomb - Regarding tracking not always "taking", one aspect here is that it is best to activate tracking from a viewpoint that is close to the viewpoints you captured the scene from. If you are doing this, we expect tracking to relocalize very quickly in most cases, but it is also true that as lighting varies a lot from the scanned model, it can be less reliable and a re-scan is important to improve it.
Over time, we will make things better so it is more stable even under more changes and bigger differences from the captured viewpoints.
However, in order to find out if the issues are related to the Unity plugin or to the underlying tech, could you try out the Bridget app and let us know if you experience the same difficulty?
PS - for your application, do you need tracking which is drift-free, or do you only need 6-DoF and drifting a bit is OK (i.e. objects will not stay perfectly fixed to the same place in the world)?