importing the GEDCOM's aren't really the problem, per se -- the problem is the original export/creation of the GEDCOM.
I've seen other people's trees on places like Ancestry...full of duplicates and other various errors of all types, so the bad data gets exported into their GEDCOM in the first place, unnoticed, then continues to be unnoticed when imported here. Ancestry has improved their error detection, but I suspect some of the errors have been around for years. And other sites probably also have loopholes in their error detection as well.
garbage in, garbage out.