For some of us, there is a substantial downside to Ancestry Tree Share and also, perhaps to a lesser degree, Family Search.
Cursory glances at the RootsMagic Forum show how much support and discussion time is devoted to the complex problems of synchronizing databases which have different structures.
I fear that the people who are programming RootsMagic are similarly burdened. Every hour devoted to massaging the connection to the Ancestry API is an hour taken away from improving the standalone versions of RootsMagic.
Many substantial enhancements to RootsMagic have been proposed, some of them very important and useful. Scroll back through the wishlist section of the forum for a samples. Are these to be delayed by the effort to work the kinks out of Tree Share?
There are also control and privacy issues. Ancestry.com is a commercial enterprise and there is always a temptation to monetize data. Many other commercial outfits have done the same, sometimes in violation of the promises they’ve made to their users. I much prefer to keep my data out of their hands; I publish what I wish via my own website (easily constructed with GedSite) and have control, or at least the illusion of greater control.
Finally, I’m sure that experienced Ancestry subscribers are aware of the pitfalls of relying on publicly posted family trees. My last adventure with such a tree posited a mother who gave birth at the age of 83. Tree Share, like Family Search, doesn’t produce that kind of research sloppiness by itself, of course, but it is likely to increase the amount of uncritical work on the web.