Merging a site with another -> galleries are all wrong afterwards

Basically, wordpress importer or exporter are doing something wrong and I can’t find, for the life of me what is it.

Site A: has more than 8000 articles into it.
Site B: has only 50 articles with galleries
Site C: hast 1500 articles with galleries

When I export Site B into A everything goes according to plan, I backup images onto Site A origin domain in /backup subdir, I modify the XML to match that site’s domain and everything imports just fine, galleries included. Everything works, pictures, addreses, featured images, all perfect.

When I export Site C into Site A everything goes to hell. Featured images are imported properly but several images are not and the galleries are all mixed up, I get a 10 picture gallery with only 2 pictures right, the rest are all garbled, like wordpress is getting the ids all wrong.

I did this with a very high performance local server, 16GB RAM, 6-core dedicated VMWARE machine with PHP memory limit set to 1GB, wait time for PHP set to 10 hours and nginx fastcgi wait to 10 hours.

The process completes perfectly, wordpress does not display any error whatsoever on the importer.

I tried changing to new users on Site A (when importing Site C) I tried to import in stages (making several XML files to reduce time) but the result is always the same, the exact same.. the galleries are a disaster, all mixed up.

I can’t find what’s wrong, because, when I import the tiny site all the galleries are correct, so, evidently, wordpress is doing something really bad when trying to merge 2 big sites.

I tried wp all import, tried several other importers but the majority wont import galleries at all, only the wordpress importer does.

I’m about to quit and call it a day, not being able to merge my site after 7 years of hard work is really horrible and the wordpress importer is now the most hated plugin I had ever used.

Please, tell me there is light at the end of the tunel.