GedSite works with GEDCOM created by RM (and also with GEDCOM produced by other software). GedSite does an excellent job of understanding RM's GEDCOM, although most other software does not do so well with RM's GEDCOM.
RM's TreeShare works with the ancestry.com API rather than with GEDCOM created by RM. TreeShare can only send things through the API that the API can understand. I don't know the details of the ancestry.com API, but the things the API can understand seems to be strongly influenced by the internal ancestry.com data model which is very different than RM's data model - for example, there are no fact notes in the ancestry.com data model and there are no shared facts in the ancestry.com data model.
Of all the things in any genealogy software that seem hard to get changed, getting the fundamental data model changed is surely one of the hardest. It's not the same thing at all (but it's really the same thing), but FamilySearch is another example of a data model that that is seriously inadequate and which I think is extremely unlikely ever to be changed.
Despite these limitations, I do think that it would be possible for the RM developers to make some major improvements to TreeShare even using the existing ancestry.com API. But I fear that doing so might require a complete "TreeShare company" with associated developers that's even bigger than the existing RM company itself.
P.S. The situation with GedSite and RM's GEDCOM is excellent but it is not perfect. I think that most of the problems are on the RM side of the house - e.g., anything that might get lost with RM's drag and drop will get lost on a RM export of GEDCOM to GedSite. This includes such things as individual customization of sentences and notes for RM's shared facts. Also, RM does not export its default sentence templates for built-in facts (I guess on the theory that if the importing software is RM then the importing software already knows what the default sentence templates for built-in facts are.) GedSite is not completely innocent, either.For example, it does not handle some of RM's sentence template options properly, for example GedSite does not handle Name part options such as :Full and :Nocycle properly (and many of the other Name part options are not handled properly).