I get a Mapping failure when importing a .mor file

Even with the exporters you could well be presented with an error box when you load a mor file saying the mapping has failed. This means that the system could not find corresponding vertices for the base morph vertices in the Unity mesh data. This can happen for various reasons.
Pivots
Try not apply any rotations, translation or scaling to the pivots of any base object or targets.
Working Pivots
Try and make sure you don’t alter the working pivots of any base mesh or targets as this will add a rotation and a translation to the morph data which will result in a mapping failure.
Exporting from FBX pose
You need to make sure that when you export the mor data that the mesh you are exporting from matches the FBX version of your mesh. For example if you are morphing an animated human character when you export to FBX the T pose mode is used but in your scene the character could be in an idle pose so the mesh data no longer matches the fbx data.
Mesh Smooth applied after Morphing
The system will not work if you have any system in place that alters the mesh after the morphing/blendshapes are calculated ie mesh smooth. As this will change the mesh that Unity sees compared to the internal morphing data that is exported.
You can also check the difference between the Unity mesh and the morph data being imported if you open the Advanced section in the inspector and click Show Mapping and set the end vert slider as required, you may also need to adjust the size value. That will show the Unity mesh vertex positions as well as the imported mapping positions, ideally they should over lap, if they don’t it will indicate what the issue could be. You can also try checking the Mapping Adjust option before importing the mor file as this will try and remove any local space scaling or translation that might be present in the data.
Lastly you can setup a very simple morph and export to get the mapping correct, once mapping is correct you can then import any mor file for the object even with scaling, rotation, offset and not matching the Unity mesh and the system will ask if you wish to use the current mapping, just click yes and the new mor file will import with no problems.