One answer may begin . . . . .
The criteria you choose to match individuals (or not),
and the completeness of that data on each (person's) record on (each of) the gedcom files(tree).
[ To define who the person is ]
I can think of a piece of software, Gedxlate, (Search Rootschat for more detail)
which can generate separate files (in say, Excel format)
Ensure the files are in the correct comparison sequence.
Then run a compare.
Unless you can find a FamilyTree application to do similar.
Sounds complicated but really is not AND there is more than one way of "x?><(*&^"ing the cat.