the mpl import did not go well at all.
The MPL import has never been able to update existing library files. Did you try with a new empty library?
IMHO, the MPL system should be developed further. It is the only way to merge separate libraries. At least it should work as before (I have not tested this yet).
An "update existing library files" option would make it much more usable. This should work quickly without checking the disk files. I have requested this a couple of times before.